diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c49f43f..cef40f1 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -27,6 +27,7 @@ jobs:
           - '8.1'
           - '8.2'
           - '8.3'
+          - '8.4'
 
     steps:
       - name: Checkout
@@ -69,4 +70,4 @@ jobs:
         if: success()
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          file: ./coverage/clover.xml
+          files: ./coverage/clover.xml
diff --git a/.php-cs-fixer.php b/.php-cs-fixer.php
index 7ba8371..2658d79 100644
--- a/.php-cs-fixer.php
+++ b/.php-cs-fixer.php
@@ -22,7 +22,7 @@
     'declare_strict_types'        => true,
     'global_namespace_import'     => false,
     'linebreak_after_opening_tag' => true,
-    'mb_str_functions'            => true,
+    'mb_str_functions'            => false, // turn off because of PHP 8.4
     'native_function_invocation'  => [
         'include' => [
             '@all'
diff --git a/src/Message/MessageTrait.php b/src/Message/MessageTrait.php
index d6a8a55..6094af1 100644
--- a/src/Message/MessageTrait.php
+++ b/src/Message/MessageTrait.php
@@ -258,7 +258,7 @@ protected function validateAndTrimHeader($header, $values): array
                 throw new \InvalidArgumentException('Header value must be RFC 7230 compatible string.');
             }
 
-            return [\trim((string) $values, " \t")];
+            return [$this->trim((string) $values, " \t")];
         }
 
         if (empty($values)) {
@@ -271,12 +271,29 @@ protected function validateAndTrimHeader($header, $values): array
                 throw new \InvalidArgumentException('Header values must be RFC 7230 compatible strings.');
             }
 
-            $returnValues[] = \trim((string) $v, " \t");
+            $returnValues[] = $this->trim((string) $v, " \t");
         }
 
         return $returnValues;
     }
 
+    /**
+     * Because of PHP 8.4.
+     *
+     * @param        $string
+     * @param string $characters
+     *
+     * @return string
+     */
+    protected function trim($string, string $characters = " \n\r\t\v\0"): string
+    {
+        if (\PHP_MAJOR_VERSION >= 8 && \PHP_MINOR_VERSION >= 4) {
+            return \mb_trim((string) $string, $characters);
+        }
+
+        return \trim((string) $string, $characters);
+    }
+
     /**
      * @param string $protocolVersion
      *
diff --git a/src/Message/Uri.php b/src/Message/Uri.php
index 18163db..422e06e 100644
--- a/src/Message/Uri.php
+++ b/src/Message/Uri.php
@@ -383,7 +383,7 @@ protected static function createUriString(
                 }
             } elseif (isset($charAtPosOne) && $charAtPosOne === '/') {
                 if ($authority === '') {
-                    $path = '/' . \ltrim($path, '/');
+                    $path = '/' . static::ltrim($path, '/');
                 }
             }
 
@@ -401,6 +401,23 @@ protected static function createUriString(
         return $uri;
     }
 
+    /**
+     * Because of PHP 8.4.
+     *
+     * @param        $string
+     * @param string $characters
+     *
+     * @return string
+     */
+    protected static function ltrim($string, string $characters = " \n\r\t\v\0"): string
+    {
+        if (\PHP_MAJOR_VERSION >= 8 && \PHP_MINOR_VERSION >= 4) {
+            return \mb_ltrim((string) $string, $characters);
+        }
+
+        return \ltrim((string) $string, $characters);
+    }
+
     /**
      * @param string $scheme
      * @param int    $port