function gstr($str)
{   
$encode = mb_detect_encoding( $str, array('ASCII','UTF-8','GB2312','GBK'));
if ( !$encode =='UTF-8' ){
$str = iconv('UTF-8',$encode,$str);
}
return $str;
}
第二种
function is_utf8_2($string) {     
    return preg_match('%^(?:  
          [\x09\x0A\x0D\x20-\x7E]            # ASCII  
        | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte  
        |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs  
        | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}  # straight 3-byte  
        |  \xED[\x80-\x9F][\x80-\xBF]        # excluding surrogates  
        |  \xF0[\x90-\xBF][\x80-\xBF]{2}     # planes 1-3  
        | [\xF1-\xF3][\x80-\xBF]{3}          # planes 4-15  
        |  \xF4[\x80-\x8F][\x80-\xBF]{2}     # plane 16  
    )*$%xs', $string);   
  
}