PHP intercepts mixed Chinese and English strings

$str='天Find我';
function cut_str($str='',$offset,$length){
	$str=iconv("utf-8","gb2312",$str);
	$str_len=strlen($str);
	$arr=array();
	for($i=0;$i<$str_len;$i++){
		if(ord($str{$i})>0x80){
			$arr[]=$str{$i}.$str{$i+1};
			$i++;
			continue;
		}
		$arr[]=$str{$i};
	}
	return array_slice($arr,$offset,$length);
}
var_dump(cut_str($str,0,2));