diff --git a/src/Symfony/Component/String/ByteString.php b/src/Symfony/Component/String/ByteString.php
index a248a5b75e..6c1bcc9e7f 100644
--- a/src/Symfony/Component/String/ByteString.php
+++ b/src/Symfony/Component/String/ByteString.php
@@ -97,7 +97,7 @@ class ByteString extends AbstractString
             $suffix = (string) $suffix;
         }
 
-        return \strlen($this->string) - \strlen($suffix) === ($this->ignoreCase ? strripos($this->string, $suffix) : strrpos($this->string, $suffix));
+        return '' !== $suffix && \strlen($this->string) >= \strlen($suffix) && 0 === substr_compare($this->string, $suffix, -\strlen($suffix), null, $this->ignoreCase);
     }
 
     public function equalsTo($string): bool
@@ -362,7 +362,7 @@ class ByteString extends AbstractString
             return parent::startsWith($prefix);
         }
 
-        return '' !== $prefix && 0 === ($this->ignoreCase ? stripos($this->string, $prefix) : strpos($this->string, $prefix));
+        return '' !== $prefix && 0 === ($this->ignoreCase ? strncasecmp($this->string, $prefix, \strlen($prefix)) : strncmp($this->string, $prefix, \strlen($prefix)));
     }
 
     public function title(bool $allWords = false): parent
diff --git a/src/Symfony/Component/String/CodePointString.php b/src/Symfony/Component/String/CodePointString.php
index e0ef21e014..6aeac3d817 100644
--- a/src/Symfony/Component/String/CodePointString.php
+++ b/src/Symfony/Component/String/CodePointString.php
@@ -93,7 +93,7 @@ class CodePointString extends AbstractUnicodeString
             return preg_match('{'.preg_quote($suffix).'$}iuD', $this->string);
         }
 
-        return \strlen($this->string) - \strlen($suffix) === strrpos($this->string, $suffix);
+        return \strlen($this->string) >= \strlen($suffix) && 0 === substr_compare($this->string, $suffix, -\strlen($suffix));
     }
 
     public function equalsTo($string): bool
@@ -107,7 +107,7 @@ class CodePointString extends AbstractUnicodeString
         }
 
         if ('' !== $string && $this->ignoreCase) {
-            return mb_strlen($string, 'UTF-8') === mb_strlen($this->string, 'UTF-8') && 0 === mb_stripos($this->string, $string, 0, 'UTF-8');
+            return \strlen($string) === \strlen($this->string) && 0 === mb_stripos($this->string, $string, 0, 'UTF-8');
         }
 
         return $string === $this->string;
@@ -256,6 +256,10 @@ class CodePointString extends AbstractUnicodeString
             return false;
         }
 
-        return 0 === ($this->ignoreCase ? mb_stripos($this->string, $prefix, 0, 'UTF-8') : strpos($this->string, $prefix));
+        if ($this->ignoreCase) {
+            return 0 === mb_stripos($this->string, $prefix, 0, 'UTF-8');
+        }
+
+        return 0 === strncmp($this->string, $prefix, \strlen($prefix));
     }
 }
diff --git a/src/Symfony/Component/String/Tests/AbstractAsciiTestCase.php b/src/Symfony/Component/String/Tests/AbstractAsciiTestCase.php
index b16bf8cc58..1e7eb4dccb 100644
--- a/src/Symfony/Component/String/Tests/AbstractAsciiTestCase.php
+++ b/src/Symfony/Component/String/Tests/AbstractAsciiTestCase.php
@@ -859,9 +859,12 @@ abstract class AbstractAsciiTestCase extends TestCase
     /**
      * @dataProvider provideStartsWith
      */
-    public function testStartsWith(bool $expected, string $origin, $prefix)
+    public function testStartsWith(bool $expected, string $origin, $prefix, int $form = null)
     {
-        $this->assertSame($expected, static::createFromString($origin)->startsWith($prefix));
+        $instance = static::createFromString($origin);
+        $instance = $form ? $instance->normalize($form) : $instance;
+
+        $this->assertSame($expected, $instance->startsWith($prefix));
     }
 
     public static function provideStartsWith()
@@ -910,9 +913,12 @@ abstract class AbstractAsciiTestCase extends TestCase
     /**
      * @dataProvider provideEndsWith
      */
-    public function testEndsWith(bool $expected, string $origin, $suffix)
+    public function testEndsWith(bool $expected, string $origin, $suffix, int $form = null)
     {
-        $this->assertSame($expected, static::createFromString($origin)->endsWith($suffix));
+        $instance = static::createFromString($origin);
+        $instance = $form ? $instance->normalize($form) : $instance;
+
+        $this->assertSame($expected, $instance->endsWith($suffix));
     }
 
     public static function provideEndsWith()
diff --git a/src/Symfony/Component/String/Tests/UnicodeStringTest.php b/src/Symfony/Component/String/Tests/UnicodeStringTest.php
index 7d9c877b44..f37476f6ce 100644
--- a/src/Symfony/Component/String/Tests/UnicodeStringTest.php
+++ b/src/Symfony/Component/String/Tests/UnicodeStringTest.php
@@ -200,4 +200,26 @@ class UnicodeStringTest extends AbstractUnicodeTestCase
             ]
         );
     }
+
+    public static function provideStartsWith()
+    {
+        return array_merge(
+            parent::provideStartsWith(),
+            [
+                [false, "cle\u{0301} prive\u{0301}e", 'cle', UnicodeString::NFD],
+                [true, "cle\u{0301} prive\u{0301}e", 'clé', UnicodeString::NFD],
+            ]
+        );
+    }
+
+    public static function provideEndsWith()
+    {
+        return array_merge(
+            parent::provideEndsWith(),
+            [
+                [false, "cle\u{0301} prive\u{0301}e", 'ee', UnicodeString::NFD],
+                [true, "cle\u{0301} prive\u{0301}e", 'ée', UnicodeString::NFD],
+            ]
+        );
+    }
 }
diff --git a/src/Symfony/Component/String/UnicodeString.php b/src/Symfony/Component/String/UnicodeString.php
index f81a9ba3ec..61e8c008dc 100644
--- a/src/Symfony/Component/String/UnicodeString.php
+++ b/src/Symfony/Component/String/UnicodeString.php
@@ -97,11 +97,15 @@ class UnicodeString extends AbstractUnicodeString
         $form = null === $this->ignoreCase ? \Normalizer::NFD : \Normalizer::NFC;
         normalizer_is_normalized($suffix, $form) ?: $suffix = normalizer_normalize($suffix, $form);
 
-        if ('' === $suffix || false === $suffix || false === $i = $this->ignoreCase ? grapheme_strripos($this->string, $suffix) : grapheme_strrpos($this->string, $suffix)) {
+        if ('' === $suffix || false === $suffix) {
             return false;
         }
 
-        return grapheme_strlen($this->string) - grapheme_strlen($suffix) === $i;
+        if ($this->ignoreCase) {
+            return 0 === mb_stripos(grapheme_extract($this->string, \strlen($suffix), GRAPHEME_EXTR_MAXBYTES, \strlen($this->string) - \strlen($suffix)), $suffix, 0, 'UTF-8');
+        }
+
+        return $suffix === grapheme_extract($this->string, \strlen($suffix), GRAPHEME_EXTR_MAXBYTES, \strlen($this->string) - \strlen($suffix));
     }
 
     public function equalsTo($string): bool
@@ -118,7 +122,7 @@ class UnicodeString extends AbstractUnicodeString
         normalizer_is_normalized($string, $form) ?: $string = normalizer_normalize($string, $form);
 
         if ('' !== $string && false !== $string && $this->ignoreCase) {
-            return grapheme_strlen($string) === grapheme_strlen($this->string) && 0 === grapheme_stripos($this->string, $string);
+            return \strlen($string) === \strlen($this->string) && 0 === mb_stripos($this->string, $string, 0, 'UTF-8');
         }
 
         return $string === $this->string;
@@ -332,7 +336,15 @@ class UnicodeString extends AbstractUnicodeString
         $form = null === $this->ignoreCase ? \Normalizer::NFD : \Normalizer::NFC;
         normalizer_is_normalized($prefix, $form) ?: $prefix = normalizer_normalize($prefix, $form);
 
-        return '' !== $prefix && false !== $prefix && 0 === ($this->ignoreCase ? grapheme_stripos($this->string, $prefix) : grapheme_strpos($this->string, $prefix));
+        if ('' === $prefix || false === $prefix) {
+            return false;
+        }
+
+        if ($this->ignoreCase) {
+            return 0 === mb_stripos(grapheme_extract($this->string, \strlen($prefix), GRAPHEME_EXTR_MAXBYTES), $prefix, 0, 'UTF-8');
+        }
+
+        return $prefix === grapheme_extract($this->string, \strlen($prefix), GRAPHEME_EXTR_MAXBYTES);
     }
 
     public function __clone()