php截取utf8或gbk編碼中英文字串

//字符串截取
$a = "s＠@你好";
var_dump(strlen_weibo($a,'utf-8'));
结果输出为8，其中字母s计数为1，全角＠计数为2，半角@计数为1，两个中文计数为4。源码如下：
//截取字符串的函数代码
function strlen_weibo($string, $charset='utf-8')
{
    $n = $count = 0;
    $length = strlen($string);
    if (strtolower($charset) == 'utf-8')
    {
        while ($n < $length)
        {
            $currentByte = ord($string[$n]);
            if ($currentByte == 9 ||
                $currentByte == 10 ||
                (32 <= $currentByte && $currentByte <= 126)) // bbs.it-home.org
            {
                $n  ;
                $count  ;
            } elseif (194 <= $currentByte && $currentByte <= 223)
            {
                $n  = 2;
                $count  = 2;
            } elseif (224 <= $currentByte && $currentByte <= 239)
            {
                $n  = 3;
                $count  = 2;
            } elseif (240 <= $currentByte && $currentByte <= 247)
            {
                $n  = 4;
                $count  = 2;
            } elseif (248 <= $currentByte && $currentByte <= 251)
            {
                $n  = 5;
                $count  = 2;
            } elseif ($currentByte == 252 || $currentByte == 253)
            {
                $n  = 6;
                $count  = 2;
            } else
            {
                $n  ;
                $count  ;
            }
            if ($count >= $length)
            {
                break;
            }
        }
        return $count;
    } else
    {
        for ($i = 0; $i < $length; $i  )
        {
            if (ord($string[$i]) > 127)
            {
                $i  ;
                $count  ;
            }
            $count  ;
        }
        return $count;
    }
}