Fix substrReplace to be multibyte safe (#57936)

* Fix substrReplace to be multibyte safe

* Apply code style fixes

* formatting

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: Nasim25

src/Illuminate/Support/Str.php

@@ -1748,10 +1748,12 @@ public static function substrCount($haystack, $needle, $offset = 0, $length = nu
     public static function substrReplace($string, $replace, $offset = 0, $length = null)
     {
         if ($length === null) {
-            $length = strlen($string);
+            $length = static::length($string);
         }
 
-        return substr_replace($string, $replace, $offset, $length);
+        return mb_substr($string, 0, $offset)
+            .$replace
+            .mb_substr($string, $offset + $length);
     }
 
     /**

tests/Support/SupportStrTest.php

@@ -1275,6 +1275,12 @@ public function testSubstrReplace()
         $this->assertSame('Laravel – The PHP Framework for Web Artisans', Str::substrReplace('Laravel Framework', '– The PHP Framework for Web Artisans', 8));
     }
 
+    public function testSubstrReplaceWithMultibyte()
+    {
+        $this->assertSame('kengä', Str::substrReplace('kenkä', 'ng', -3, 2));
+        $this->assertSame('kenga', Str::substrReplace('kenka', 'ng', -3, 2));
+    }
+
     public function testTake()
     {
         $this->assertSame('ab', Str::take('abcdef', 2));