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;
  }