PHP如何将不是UTF8的字符过滤掉（代码）

function utf8_filter($data)
  {
     $str = "";
     for($n = 0; $n < strlen($data);)
     {
          $s = substr($data, $n, 1);
          $v = ord($s);
          if($v >= 127)
          {
             ++$n;
             $cnt = 0;
             $tmp = $v;
             while($tmp & 0x80)
             {
                $tmp = $tmp << 1;
                ++$cnt;
              }
              $x = 0;
              while($x < $cnt && $n < strlen($data))
              {
                $s = substr($data, $n, 1);
                if((ord($s) & 0xC0) == 0x80)
                {
                   ++$n;
                   ++$x;
                 }else{
                         break;
                        }
                  }
                 if($x + 1 == $cnt)
                 {
                    $str  = $str . substr($data, $n - $cnt, $cnt);
                  }else{
                         while($n < strlen($data))
                        {
                          $s = substr($data, $n, 1);
                          if(ord($s) & 0x80)
                          {
                             ++$n;
                           }else{
                                    break;
                                 }
                            }
          }
         }else{
       $str = $str. $s;
       ++$n;
      }
}
return  $str;
}