diff --git a/src/Symfony/Component/DependencyInjection/CHANGELOG.md b/src/Symfony/Component/DependencyInjection/CHANGELOG.md
index 1fc26be27e..49e140de11 100644
--- a/src/Symfony/Component/DependencyInjection/CHANGELOG.md
+++ b/src/Symfony/Component/DependencyInjection/CHANGELOG.md
@@ -7,6 +7,7 @@ CHANGELOG
  * deprecated service auto-registration while autowiring
  * deprecated the ability to check for the initialization of a private service with the `Container::initialized()` method
  * deprecated support for top-level anonymous services in XML
+ * deprecated case insensitivity of parameter names
 
 3.3.0
 -----
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index 52a9f8aca8..49cd09f02a 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -1056,11 +1056,15 @@ EOF;
 
         $php = array();
         $dynamicPhp = array();
+        $normalizedParams = array();
 
         foreach ($this->container->getParameterBag()->all() as $key => $value) {
             if ($key !== $resolvedKey = $this->container->resolveEnvPlaceholders($key)) {
                 throw new InvalidArgumentException(sprintf('Parameter name cannot use env parameters: %s.', $resolvedKey));
             }
+            if ($key !== $lcKey = strtolower($key)) {
+                $normalizedParams[] = sprintf('        %s => %s,', $this->export($lcKey), $this->export($key));
+            }
             $export = $this->exportParameters(array($value));
             $export = explode('0 => ', substr(rtrim($export, " )\n"), 7, -1), 2);
 
@@ -1082,7 +1086,7 @@ EOF;
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -1100,7 +1104,7 @@ EOF;
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -1170,6 +1174,26 @@ EOF;
 {$getDynamicParameter}
     }
 
+
+EOF;
+
+            $code .= '    private $normalizedParameterNames = '.($normalizedParams ? sprintf("array(\n%s\n    );", implode("\n", $normalizedParams)) : 'array();')."\n";
+            $code .= <<<'EOF'
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
 EOF;
         } elseif ($dynamicPhp) {
             throw new RuntimeException('You cannot dump a not-frozen container with dynamic parameters.');
@@ -1558,10 +1582,10 @@ EOF;
             if (preg_match('/^%([^%]+)%$/', $value, $match)) {
                 // we do this to deal with non string values (Boolean, integer, ...)
                 // the preg_replace_callback converts them to strings
-                return $this->dumpParameter(strtolower($match[1]));
+                return $this->dumpParameter($match[1]);
             } else {
                 $replaceParameters = function ($match) {
-                    return "'.".$this->dumpParameter(strtolower($match[2])).".'";
+                    return "'.".$this->dumpParameter($match[2]).".'";
                 };
 
                 $code = str_replace('%%', '%', preg_replace_callback('/(?<!%)(%)([^%]+)\1/', $replaceParameters, $this->export($value)));
@@ -1607,8 +1631,6 @@ EOF;
      */
     private function dumpParameter($name)
     {
-        $name = strtolower($name);
-
         if ($this->container->isCompiled() && $this->container->hasParameter($name)) {
             $value = $this->container->getParameter($name);
             $dumpedValue = $this->dumpValue($value, false);
diff --git a/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
index f5433bb6ba..0a5f9593d4 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
@@ -486,11 +486,6 @@ class XmlFileLoader extends FileLoader
                 $key = array_pop($keys);
             } else {
                 $key = $arg->getAttribute('key');
-
-                // parameter keys are case insensitive
-                if ('parameter' == $name && $lowercase) {
-                    $key = strtolower($key);
-                }
             }
 
             $onInvalid = $arg->getAttribute('on-invalid');
diff --git a/src/Symfony/Component/DependencyInjection/ParameterBag/EnvPlaceholderParameterBag.php b/src/Symfony/Component/DependencyInjection/ParameterBag/EnvPlaceholderParameterBag.php
index d20e53531a..fd859068be 100644
--- a/src/Symfony/Component/DependencyInjection/ParameterBag/EnvPlaceholderParameterBag.php
+++ b/src/Symfony/Component/DependencyInjection/ParameterBag/EnvPlaceholderParameterBag.php
@@ -91,7 +91,7 @@ class EnvPlaceholderParameterBag extends ParameterBag
         parent::resolve();
 
         foreach ($this->envPlaceholders as $env => $placeholders) {
-            if (!isset($this->parameters[$name = strtolower("env($env)")])) {
+            if (!$this->has($name = "env($env)")) {
                 continue;
             }
             if (is_numeric($default = $this->parameters[$name])) {
diff --git a/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php b/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
index 83ba7e7076..1a9ca6fa4d 100644
--- a/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
+++ b/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
@@ -25,6 +25,8 @@ class ParameterBag implements ParameterBagInterface
     protected $parameters = array();
     protected $resolved = false;
 
+    private $normalizedNames = array();
+
     /**
      * @param array $parameters An array of parameters
      */
@@ -49,7 +51,7 @@ class ParameterBag implements ParameterBagInterface
     public function add(array $parameters)
     {
         foreach ($parameters as $key => $value) {
-            $this->parameters[strtolower($key)] = $value;
+            $this->set($key, $value);
         }
     }
 
@@ -66,7 +68,7 @@ class ParameterBag implements ParameterBagInterface
      */
     public function get($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeName($name);
 
         if (!array_key_exists($name, $this->parameters)) {
             if (!$name) {
@@ -111,7 +113,7 @@ class ParameterBag implements ParameterBagInterface
      */
     public function set($name, $value)
     {
-        $this->parameters[strtolower($name)] = $value;
+        $this->parameters[$this->normalizeName($name)] = $value;
     }
 
     /**
@@ -119,7 +121,7 @@ class ParameterBag implements ParameterBagInterface
      */
     public function has($name)
     {
-        return array_key_exists(strtolower($name), $this->parameters);
+        return array_key_exists($this->normalizeName($name), $this->parameters);
     }
 
     /**
@@ -129,7 +131,7 @@ class ParameterBag implements ParameterBagInterface
      */
     public function remove($name)
     {
-        unset($this->parameters[strtolower($name)]);
+        unset($this->parameters[$this->normalizeName($name)]);
     }
 
     /**
@@ -206,7 +208,7 @@ class ParameterBag implements ParameterBagInterface
         // a non-string in a parameter value
         if (preg_match('/^%([^%\s]+)%$/', $value, $match)) {
             $key = $match[1];
-            $lcKey = strtolower($key);
+            $lcKey = strtolower($key); // strtolower() to be removed in 4.0
 
             if (isset($resolving[$lcKey])) {
                 throw new ParameterCircularReferenceException(array_keys($resolving));
@@ -224,7 +226,7 @@ class ParameterBag implements ParameterBagInterface
             }
 
             $key = $match[1];
-            $lcKey = strtolower($key);
+            $lcKey = strtolower($key); // strtolower() to be removed in 4.0
             if (isset($resolving[$lcKey])) {
                 throw new ParameterCircularReferenceException(array_keys($resolving));
             }
@@ -288,4 +290,18 @@ class ParameterBag implements ParameterBagInterface
 
         return $value;
     }
+
+    private function normalizeName($name)
+    {
+        if (isset($this->normalizedNames[$normalizedName = strtolower($name)])) {
+            $normalizedName = $this->normalizedNames[$normalizedName];
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
 }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
index efe9840e42..895e035185 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
@@ -1129,6 +1129,21 @@ class ContainerBuilderTest extends TestCase
 
         $container->get('bar');
     }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "FOO" instead of "foo" is deprecated since version 3.4.
+     */
+    public function testParameterWithMixedCase()
+    {
+        $container = new ContainerBuilder(new ParameterBag(array('foo' => 'bar')));
+        $container->register('foo', 'stdClass')
+            ->setProperty('foo', '%FOO%');
+
+        $container->compile();
+
+        $this->assertSame('bar', $container->get('foo')->foo);
+    }
 }
 
 class FooClass
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ContainerTest.php b/src/Symfony/Component/DependencyInjection/Tests/ContainerTest.php
index 75435cf568..d49df4ec42 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ContainerTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ContainerTest.php
@@ -125,10 +125,6 @@ class ContainerTest extends TestCase
         $sc->setParameter('foo', 'baz');
         $this->assertEquals('baz', $sc->getParameter('foo'), '->setParameter() overrides previously set parameter');
 
-        $sc->setParameter('Foo', 'baz1');
-        $this->assertEquals('baz1', $sc->getParameter('foo'), '->setParameter() converts the key to lowercase');
-        $this->assertEquals('baz1', $sc->getParameter('FOO'), '->getParameter() converts the key to lowercase');
-
         try {
             $sc->getParameter('baba');
             $this->fail('->getParameter() thrown an \InvalidArgumentException if the key does not exist');
@@ -138,6 +134,20 @@ class ContainerTest extends TestCase
         }
     }
 
+    /**
+     * @group legacy
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "Foo" instead of "foo" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "FOO" instead of "foo" is deprecated since version 3.4.
+     */
+    public function testGetSetParameterWithMixedCase()
+    {
+        $sc = new Container(new ParameterBag(array('foo' => 'bar')));
+
+        $sc->setParameter('Foo', 'baz1');
+        $this->assertEquals('baz1', $sc->getParameter('foo'), '->setParameter() converts the key to lowercase');
+        $this->assertEquals('baz1', $sc->getParameter('FOO'), '->getParameter() converts the key to lowercase');
+    }
+
     public function testGetServiceIds()
     {
         $sc = new Container();
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Dumper/PhpDumperTest.php b/src/Symfony/Component/DependencyInjection/Tests/Dumper/PhpDumperTest.php
index e60ee91cdd..338d7b5bb3 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Dumper/PhpDumperTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Dumper/PhpDumperTest.php
@@ -670,4 +670,43 @@ class PhpDumperTest extends TestCase
 
         $container->get('bar');
     }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "foo" instead of "Foo" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "FOO" instead of "Foo" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "bar" instead of "BAR" is deprecated since version 3.4.
+     */
+    public function testParameterWithMixedCase()
+    {
+        $container = new ContainerBuilder(new ParameterBag(array('Foo' => 'bar', 'BAR' => 'foo')));
+        $container->compile();
+
+        $dumper = new PhpDumper($container);
+        eval('?>'.$dumper->dump(array('class' => 'Symfony_DI_PhpDumper_Test_Parameter_With_Mixed_Case')));
+
+        $container = new \Symfony_DI_PhpDumper_Test_Parameter_With_Mixed_Case();
+
+        $this->assertSame('bar', $container->getParameter('foo'));
+        $this->assertSame('bar', $container->getParameter('FOO'));
+        $this->assertSame('foo', $container->getParameter('bar'));
+        $this->assertSame('foo', $container->getParameter('BAR'));
+    }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "FOO" instead of "foo" is deprecated since version 3.4.
+     */
+    public function testParameterWithLowerCase()
+    {
+        $container = new ContainerBuilder(new ParameterBag(array('foo' => 'bar')));
+        $container->compile();
+
+        $dumper = new PhpDumper($container);
+        eval('?>'.$dumper->dump(array('class' => 'Symfony_DI_PhpDumper_Test_Parameter_With_Lower_Case')));
+
+        $container = new \Symfony_DI_PhpDumper_Test_Parameter_With_Lower_Case();
+
+        $this->assertSame('bar', $container->getParameter('FOO'));
+    }
 }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/containers/container8.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/containers/container8.php
index f0b4ca9330..1a4e5ab5c8 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/containers/container8.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/containers/container8.php
@@ -4,7 +4,7 @@ use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\ParameterBag\ParameterBag;
 
 $container = new ContainerBuilder(new ParameterBag(array(
-    'FOO' => '%baz%',
+    'foo' => '%baz%',
     'baz' => 'bar',
     'bar' => 'foo is %%foo bar',
     'escape' => '@escapeme',
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services10.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services10.php
index 19fe020694..d8478d677f 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services10.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services10.php
@@ -78,7 +78,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -96,7 +96,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -142,6 +142,22 @@ class ProjectServiceContainer extends Container
         throw new InvalidArgumentException(sprintf('The dynamic parameter "%s" must be defined.', $name));
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services12.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services12.php
index 77224a5b8f..976c720768 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services12.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services12.php
@@ -82,7 +82,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -100,7 +100,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -156,6 +156,22 @@ class ProjectServiceContainer extends Container
         return $this->dynamicParameters[$name] = $value;
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services26.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services26.php
index 5ec14c1026..0998b23a75 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services26.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services26.php
@@ -80,7 +80,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -98,7 +98,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -152,6 +152,24 @@ class ProjectServiceContainer extends Container
         return $this->dynamicParameters[$name] = $value;
     }
 
+    private $normalizedParameterNames = array(
+        'env(foo)' => 'env(FOO)',
+    );
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
@@ -160,7 +178,7 @@ class ProjectServiceContainer extends Container
     protected function getDefaultParameters()
     {
         return array(
-            'env(foo)' => 'foo',
+            'env(FOO)' => 'foo',
         );
     }
 }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services8.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services8.php
index f85e3251be..1689a96586 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services8.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services8.php
@@ -65,7 +65,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -83,7 +83,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -129,6 +129,22 @@ class ProjectServiceContainer extends Container
         throw new InvalidArgumentException(sprintf('The dynamic parameter "%s" must be defined.', $name));
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_as_files.txt b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_as_files.txt
index 3b80a904d3..2793610bae 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_as_files.txt
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_as_files.txt
@@ -338,7 +338,7 @@ class Container%s extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -356,7 +356,7 @@ class Container%s extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -402,6 +402,22 @@ class Container%s extends Container
         throw new InvalidArgumentException(sprintf('The dynamic parameter "%s" must be defined.', $name));
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_compiled.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_compiled.php
index b01733cabf..e77bc56579 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_compiled.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_compiled.php
@@ -341,7 +341,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -359,7 +359,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -405,6 +405,22 @@ class ProjectServiceContainer extends Container
         throw new InvalidArgumentException(sprintf('The dynamic parameter "%s" must be defined.', $name));
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services_array_params.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services_array_params.php
index b8d52d69f5..d848941013 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services_array_params.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services_array_params.php
@@ -86,7 +86,7 @@ class ProjectServiceContainer extends Container
     public function getParameter($name)
     {
         if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
-            $name = strtolower($name);
+            $name = $this->normalizeParameterName($name);
 
             if (!(isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters))) {
                 throw new InvalidArgumentException(sprintf('The parameter "%s" must be defined.', $name));
@@ -104,7 +104,7 @@ class ProjectServiceContainer extends Container
      */
     public function hasParameter($name)
     {
-        $name = strtolower($name);
+        $name = $this->normalizeParameterName($name);
 
         return isset($this->parameters[$name]) || isset($this->loadedDynamicParameters[$name]) || array_key_exists($name, $this->parameters);
     }
@@ -160,6 +160,22 @@ class ProjectServiceContainer extends Container
         return $this->dynamicParameters[$name] = $value;
     }
 
+    private $normalizedParameterNames = array();
+
+    private function normalizeParameterName($name)
+    {
+        if (isset($this->normalizedParameterNames[$normalizedName = strtolower($name)]) || isset($this->parameters[$normalizedName]) || array_key_exists($normalizedName, $this->parameters)) {
+            $normalizedName = isset($this->normalizedParameterNames[$normalizedName]) ? $this->normalizedParameterNames[$normalizedName] : $normalizedName;
+            if ((string) $name !== $normalizedName) {
+                @trigger_error(sprintf('Parameter names will be made case sensitive in Symfony 4.0. Using "%s" instead of "%s" is deprecated since version 3.4.', $name, $normalizedName), E_USER_DEPRECATED);
+            }
+        } else {
+            $normalizedName = $this->normalizedParameterNames[$normalizedName] = (string) $name;
+        }
+
+        return $normalizedName;
+    }
+
     /**
      * Gets the default parameters.
      *
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/services2.xml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/services2.xml
index f1ac14dd5d..0d2d669949 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/services2.xml
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/services2.xml
@@ -5,7 +5,7 @@
     xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
   <parameters>
     <parameter>a string</parameter>
-    <parameter key="FOO">bar</parameter>
+    <parameter key="foo">bar</parameter>
     <parameter key="values" type="collection">
       <parameter>0</parameter>
       <parameter key="integer">4</parameter>
@@ -23,7 +23,7 @@
         <parameter>bar</parameter>
       </parameter>
     </parameter>
-    <parameter key="MixedCase" type="collection"> <!-- Should be lower cased -->
+    <parameter key="mixedcase" type="collection"> <!-- Should be lower cased -->
       <parameter key="MixedCaseKey">value</parameter> <!-- Should stay mixed case -->
     </parameter>
     <parameter key="constant" type="constant">PHP_EOL</parameter>
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/services2.yml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/services2.yml
index 7ab302bcb9..e05d69d77e 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/services2.yml
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/services2.yml
@@ -1,5 +1,5 @@
 parameters:
-    FOO: bar
+    foo: bar
     values:
         - true
         - false
@@ -9,5 +9,5 @@ parameters:
     bar: foo
     escape: '@@escapeme'
     foo_bar: '@foo_bar'
-    MixedCase:
+    mixedcase:
         MixedCaseKey: value
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/EnvPlaceholderParameterBagTest.php b/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/EnvPlaceholderParameterBagTest.php
index 01fcd2c3ef..9abfb45d6e 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/EnvPlaceholderParameterBagTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/EnvPlaceholderParameterBagTest.php
@@ -115,18 +115,18 @@ class EnvPlaceholderParameterBagTest extends TestCase
     {
         $bag = new EnvPlaceholderParameterBag();
         $bag->get('env(INT_VAR)');
-        $bag->set('env(Int_Var)', 2);
+        $bag->set('env(INT_VAR)', 2);
         $bag->resolve();
-        $this->assertSame('2', $bag->all()['env(int_var)']);
+        $this->assertSame('2', $bag->all()['env(INT_VAR)']);
     }
 
     public function testResolveEnvAllowsNull()
     {
         $bag = new EnvPlaceholderParameterBag();
         $bag->get('env(NULL_VAR)');
-        $bag->set('env(Null_Var)', null);
+        $bag->set('env(NULL_VAR)', null);
         $bag->resolve();
-        $this->assertNull($bag->all()['env(null_var)']);
+        $this->assertNull($bag->all()['env(NULL_VAR)']);
     }
 
     /**
@@ -137,7 +137,7 @@ class EnvPlaceholderParameterBagTest extends TestCase
     {
         $bag = new EnvPlaceholderParameterBag();
         $bag->get('env(ARRAY_VAR)');
-        $bag->set('env(Array_Var)', array());
+        $bag->set('env(ARRAY_VAR)', array());
         $bag->resolve();
     }
 
@@ -148,7 +148,7 @@ class EnvPlaceholderParameterBagTest extends TestCase
         $bag->get('env(NULL_VAR)');
         $bag->resolve();
 
-        $this->assertNull($bag->all()['env(null_var)']);
+        $this->assertNull($bag->all()['env(NULL_VAR)']);
     }
 
     /**
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/ParameterBagTest.php b/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/ParameterBagTest.php
index 391e0eb3ff..3bac9ca64e 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/ParameterBagTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ParameterBag/ParameterBagTest.php
@@ -46,8 +46,6 @@ class ParameterBagTest extends TestCase
         ));
         $bag->remove('foo');
         $this->assertEquals(array('bar' => 'bar'), $bag->all(), '->remove() removes a parameter');
-        $bag->remove('BAR');
-        $this->assertEquals(array(), $bag->all(), '->remove() converts key to lowercase before removing');
     }
 
     public function testGetSet()
@@ -59,10 +57,6 @@ class ParameterBagTest extends TestCase
         $bag->set('foo', 'baz');
         $this->assertEquals('baz', $bag->get('foo'), '->set() overrides previously set parameter');
 
-        $bag->set('Foo', 'baz1');
-        $this->assertEquals('baz1', $bag->get('foo'), '->set() converts the key to lowercase');
-        $this->assertEquals('baz1', $bag->get('FOO'), '->get() converts the key to lowercase');
-
         try {
             $bag->get('baba');
             $this->fail('->get() throws an Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException if the key does not exist');
@@ -109,10 +103,33 @@ class ParameterBagTest extends TestCase
     {
         $bag = new ParameterBag(array('foo' => 'bar'));
         $this->assertTrue($bag->has('foo'), '->has() returns true if a parameter is defined');
-        $this->assertTrue($bag->has('Foo'), '->has() converts the key to lowercase');
         $this->assertFalse($bag->has('bar'), '->has() returns false if a parameter is not defined');
     }
 
+    /**
+     * @group legacy
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "BAR" instead of "bar" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "Foo" instead of "foo" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "FOO" instead of "foo" is deprecated since version 3.4.
+     * @expectedDeprecation Parameter names will be made case sensitive in Symfony 4.0. Using "Foo" instead of "foo" is deprecated since version 3.4.
+     */
+    public function testMixedCase()
+    {
+        $bag = new ParameterBag(array(
+            'foo' => 'foo',
+            'bar' => 'bar',
+        ));
+
+        $bag->remove('BAR');
+        $this->assertEquals(array('foo' => 'foo'), $bag->all(), '->remove() converts key to lowercase before removing');
+
+        $bag->set('Foo', 'baz1');
+        $this->assertEquals('baz1', $bag->get('foo'), '->set() converts the key to lowercase');
+        $this->assertEquals('baz1', $bag->get('FOO'), '->get() converts the key to lowercase');
+
+        $this->assertTrue($bag->has('Foo'), '->has() converts the key to lowercase');
+    }
+
     public function testResolveValue()
     {
         $bag = new ParameterBag(array());