eZ\Publish\Core\Persistence\Utf8Converter::toUTF8Character PHP Method

    public static function toUTF8Character($charCode)
    {
        switch ($charCode) {
            case 0:
                $char = chr(0);
                break;
            case !($charCode & 0xffffff80):
                // 7 bit
                $char = chr($charCode);
                break;
            case !($charCode & 0xfffff800):
                // 11 bit
                $char = chr(0xc0 | $charCode >> 6 & 0x1f) . chr(0x80 | $charCode & 0x3f);
                break;
            case !($charCode & 0xffff0000):
                // 16 bit
                $char = chr(0xe0 | $charCode >> 12 & 0xf) . chr(0x80 | $charCode >> 6 & 0x3f) . chr(0x80 | $charCode & 0x3f);
                break;
            case !($charCode & 0xffe00000):
                // 21 bit
                $char = chr(0xf0 | $charCode >> 18 & 0x7) . chr(0x80 | $charCode >> 12 & 0x3f) . chr(0x80 | $charCode >> 6 & 0x3f) . chr(0x80 | $charCode & 0x3f);
                break;
            case !($charCode & 0xfc000000):
                // 26 bit
                $char = chr(0xf8 | $charCode >> 24 & 0x3) . chr(0x80 | $charCode >> 18 & 0x3f) . chr(0x80 | $charCode >> 12 & 0x3f) . chr(0x80 | $charCode >> 6 & 0x3f) . chr(0x80 | $charCode & 0x3f);
                break;
            default:
                // 31 bit
                $char = chr(0xfc | $charCode >> 30 & 0x1) . chr(0x80 | $charCode >> 24 & 0x3f) . chr(0x80 | $charCode >> 18 & 0x3f) . chr(0x80 | $charCode >> 12 & 0x3f) . chr(0x80 | $charCode >> 6 & 0x3f) . chr(0x80 | $charCode & 0x3f);
        }
        return $char;
    }

 /**
  * Compile a single source character definition into a plain UTF-8 character
  *
  * Handles the two formats from the possible character definitions:
  *  - U+xxxx : Unicode value in hexadecimal
  *  - xx: Ascii value in hexadecimal
  *
  * @param string $char
  *
  * @return string
  */
 protected function compileCharacter($char)
 {
     switch (true) {
         case preg_match('(^U\\+[0-9a-fA-F]{4}$)', $char):
             return $this->converter->toUTF8Character(hexdec(substr($char, 2)));
         case preg_match('(^[0-9a-fA-F]{2}$)', $char):
             return chr(hexdec($char));
         default:
             throw new RuntimeException("Invalid character definition: {$char}");
     }
 }