php检测文本的编码

public function chkCode($string) {
    $code = array(
      "ASCII",
      "GBK",
      "UTF-8"
    );
    foreach ($code as $c) {
      if ($string === iconv("UTF-8", $c, iconv($c, "UTF-8", $string))) {
        return $c;
      }
    }
    return null;
  }