php-PHP中mysql_num_rows问题

 alert('请确认信息完整性！'); history.go(-1);";        }        else        {            if($psw == $psw_confirm)            {                mysql_connect("localhost","root","");   //连接数据库                mysql_select_db("用户表");   //选择数据库                mysql_query("set names 'gdk'"); //设定字符集                $sql = "select username from user where username = '$_POST[username]'"; //SQL语句                $result = mysql_query($sql);    //执行SQL语句                $num = mysql_num_rows($result);

 alert('请输入用户名或密码！'); history.go(-1);";        }        else        {            mysql_connect("localhost","root","");            mysql_select_db("用户表");            mysql_query("set names 'gbk'");            $sql = "select username,password from user where username = '$_POST[username]' and password = '$_POST[password]'";            $result = mysql_query($sql);            $num = mysql_num_rows($result);