How to filter out characters that are not UTF8 in PHP (code)

The content of this article is about how PHP filters out characters that are not UTF8 (code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

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;
}

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