函数内不能用为什么

Question

<?php
include_once("conn/classDB.php");
include_once("conn/conn.php"); $db = new class_mysql();
         $ads_info = "SELECT count(id) as counts FROM guanggao";
$ads_infos = $db->getCounts($ads_info);
echo $ads_infos;?>
     这个可以打印出结果但是我把它封装到类里面就没有了结果了 这个是为什么啊，是怎么回事呢
<?php include_once("conn/classDB.php");
 include_once("conn/conn.php");  $db = new class_mysql();
    class GuangGaoManiger { function  getAllCount()
{
//global $db;
//$db = new class_mysql();
$ads_info = "SELECT count(id) as counts FROM guanggao";
$ads_infos = $db->getCounts($ads_info);
echo $ads_infos;
return $ads_infos ;
}
}

Answer 1

echo $ads_infos; 
去掉这句话

Answer 2

function  getAllCount()
{
//global $db;
//$db = new class_mysql();
$ads_info = "SELECT count(id) as counts FROM guanggao";
$ads_infos = $this->db->getCounts($ads_info);
echo $ads_infos;
return $ads_infos ;
}
}