php intercept function containing mixed string lengths

 It must be possible for Chinese, English and special characters.
//utf-8 Chinese interception, single-byte interception mode
function cn_substr_utf8($str,$length,$append='...',$start=0){
 if(strlen($str)<$ start+1){
 return '';
 }
 preg_match_all("/./su",$str,$ar);
 $str2='';
 $tstr='';
 for($i=0 ;isset($ar[0][$i]);$i++){
 if(strlen($tstr)<$start){
 $tstr.=$ar[0][$i];
 }else {
 if(strlen($str2)<$length + strlen($ar[0][$i])){
 $str2.=$ar[0][$i];
 }else{
 break;
 }
 }
 }
 return $str==$str2?$str2:$str2.$append;
}
//----Usage examples are as follows:
echo cn_substr_utf8('I am a, and ha, oh Also,,country!',12);

php