$str = "abcdef啊啊吧啊";

function my_sub($str, $st ,$len){

    $ret = "";

    for( $st; $len > 0; $st++){

        $cStr = $str[$st];

        if( ord( $cStr ) > 127 ){

            $cStr = substr($str, $st, 3);

            $st += 2;

        }

        $len -= 1;

        $ret .= $cStr;

    }

    return $ret;

}

 echo my_sub($str, 3, 6);