diff --git a/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php b/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
index eef3f62a06..6263267fef 100644
--- a/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
+++ b/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
@@ -247,7 +247,7 @@ class Deprecation
             foreach (get_declared_classes() as $class) {
                 if ('C' === $class[0] && 0 === strpos($class, 'ComposerAutoloaderInit')) {
                     $r = new \ReflectionClass($class);
-                    $v = \dirname($r->getFileName(), 2);
+                    $v = \dirname(\dirname($r->getFileName()));
                     if (file_exists($v.'/composer/installed.json')) {
                         self::$vendors[] = $v;
                         $loader = require $v.'/autoload.php';
diff --git a/src/Symfony/Bridge/PhpUnit/Tests/DeprecationErrorHandler/DeprecationTest.php b/src/Symfony/Bridge/PhpUnit/Tests/DeprecationErrorHandler/DeprecationTest.php
index 7ff480fec3..d59b2d9337 100644
--- a/src/Symfony/Bridge/PhpUnit/Tests/DeprecationErrorHandler/DeprecationTest.php
+++ b/src/Symfony/Bridge/PhpUnit/Tests/DeprecationErrorHandler/DeprecationTest.php
@@ -29,7 +29,7 @@ class DeprecationTest extends TestCase
     {
         $r = new \ReflectionClass(Deprecation::class);
 
-        if (dirname($r->getFileName(), 2) !== dirname(__DIR__, 2)) {
+        if (\dirname(\dirname($r->getFileName())) !== \dirname(\dirname(__DIR__))) {
             $this->markTestSkipped('Test case is not compatible with having the bridge in vendor/');
         }