[mbstring][PHP 8.4] Add mb_ucfirst and mb_lcfirst to polyfills

Adds polyfills for `mb_ucfirst` and `mb_lcfirst` functions based on
the polyfill shown in PHP.Watch. It basically takes the first mb
character, calls `mb_convert_case` with `MB_CASE_TITLE` or
`MB_CASE_LOWER`, and returns with the concat of the remaining string.

The tests are taken from the php-src PR.

 - [RFC](https://wiki.php.net/rfc/mb_ucfirst)
 - [php-src PR: php-src#13161](php/php-src#13161)
 - [PHP.Watch - PHP 8.4: New `mb_ucfirst` and `mb_lcfirst` functions](https://php.watch/versions/8.4/mb_ucfirst-mb_ucfirst)

Co-authored-by: USAMI Kenta <tadsan@zonu.me>

Php84.php

@@ -18,4 +18,49 @@
  */
 final class Php84
 {
+    public static function mb_ucfirst(string $string, ?string $encoding = null): string
+    {
+        if (null === $encoding) {
+            $encoding = mb_internal_encoding();
+        }
+
+        try {
+            $validEncoding = @mb_check_encoding('', $encoding);
+        } catch (\ValueError $e) {
+            throw new \ValueError(sprintf('mb_ucfirst(): Argument #2 ($encoding) must be a valid encoding, "%s" given', $encoding));
+        }
+
+        // BC for PHP 7.3 and lower
+        if (!$validEncoding) {
+            throw new \ValueError(sprintf('mb_ucfirst(): Argument #2 ($encoding) must be a valid encoding, "%s" given', $encoding));
+        }
+
+        $firstChar = mb_substr($string, 0, 1, $encoding);
+        $firstChar = mb_convert_case($firstChar, MB_CASE_TITLE, $encoding);
+
+        return $firstChar . mb_substr($string, 1, null, $encoding);
+    }
+
+    public static function mb_lcfirst(string $string, ?string $encoding = null): string
+    {
+        if (null === $encoding) {
+            $encoding = mb_internal_encoding();
+        }
+
+        try {
+            $validEncoding = @mb_check_encoding('', $encoding);
+        } catch (\ValueError $e) {
+            throw new \ValueError(sprintf('mb_lcfirst(): Argument #2 ($encoding) must be a valid encoding, "%s" given', $encoding));
+        }
+
+        // BC for PHP 7.3 and lower
+        if (!$validEncoding) {
+            throw new \ValueError(sprintf('mb_lcfirst(): Argument #2 ($encoding) must be a valid encoding, "%s" given', $encoding));
+        }
+
+        $firstChar = mb_substr($string, 0, 1, $encoding);
+        $firstChar = mb_convert_case($firstChar, MB_CASE_LOWER, $encoding);
+
+        return $firstChar . mb_substr($string, 1, null, $encoding);
+    }
 }

README.md

@@ -3,6 +3,8 @@ Symfony Polyfill / Php84
 
 This component provides features added to PHP 8.4 core:
 
+- [`mb_ucfirst` and `mb_lcfirst`](https://wiki.php.net/rfc/mb_ucfirst)
+
 More information can be found in the
 [main Polyfill README](https://github.com/symfony/polyfill/blob/main/README.md).
 

bootstrap.php

@@ -14,3 +14,12 @@
 if (\PHP_VERSION_ID >= 80400) {
     return;
 }
+
+
+if (!function_exists('mb_ucfirst')) {
+    function mb_ucfirst($string, ?string $encoding = null): string { return p\Php84::mb_ucfirst($string, $encoding); }
+}
+
+if (!function_exists('mb_lcfirst')) {
+    function mb_lcfirst($string, ?string $encoding = null): string { return p\Php84::mb_lcfirst($string, $encoding); }
+}