没事写了两个函数，php中gbk和utf-8中文无乱码截取

//gbk,gb2312字符串截取函数
function gbk_substr($str,$len){
	if($len<=0)
		return '';
	$chars=0;
	$bytes=0;
	$str_bytes=strlen($str);
	for($i=0;$i<$str_bytes && $chars<$len;$chars++){
		if(ord($str[$i])>127){//当字符的码值大于127时为中文,此时应该截取两个字节
			$bytes += 2;
			$i += 2;
		}else{
			$bytes += 1;
			$i += 1;
		}
	}
	return substr($str,0,$bytes);
}

//utf-8字符串截取函数
function utf8_substr($str,$len){
	if($len<=0)
		return '';
	$chars=0;
	$bytes=0;
	$str_bytes=strlen($str);
	for($i=0;$i<$str_bytes && $chars<$len;$chars++){
		$bin_str=decbin(ord($str[$i]));
		
	if(strlen($bin_str)<8){
           $bytes += 1;
	   $i += 1;
        }else if(($bin_str & "11100000")=="11000000"){
           $bytes += 2;
	   $i += 2;
        }else if(($bin_str & "11110000")=="11100000"){
           $bytes += 3;
	   $i += 3;
        }else if(($bin_str & '11111000')=='11110000'){
            $bytes += 4;
	   $i += 4;
        }else if(($bin_str & "11111100")=="11111000"){
            $bytes += 5;
	   $i += 5;
        }else if(($bin_str & "11111110")=="11111100"){
            $bytes += 6;
	   $i += 6;
        }
		
	}
	
	return substr($str,0,$bytes);
}