使用PHP PDO问题检查表中的条目

我想测试user_saved表中是否存在特定UID和MID的条目，下面是我所做的

$result = $db->prepare("SELECT * FROM user_saved WHERE UID = ? AND MID = ?");   
          $result->execute(array($_SESSION['uid'], $itemId['MID2']));
          $test = $result->fetch(PDO::FETCH_ASSOC); 
                if (isset($test)){
        exist
      } else {
                    doesn't exist
              }

问题是，我尝试使用一个不存在的UID和MID，但它总是进入isset if语句，这是为什么？有什么更好的方法可以做到这一点

---

isset检查变量是否存在使用strlen函数或数组计数函数

<?php
$bbnq = sprintf("SELECT login
FROM users
WHERE id = %u",27);
try
    { $req = $db_bbn->query($bbnq); }
catch (Exception $e)
    { bbnf_pdo_error($e,__FILE__,__LINE__); }
if ( $r = $req->fetch() )
    { echo "This query has a row result"; }
else
    { echo "This query has an empty result"; }
?>

<?php
$bbnq = sprintf("SELECT login
FROM users
WHERE id = %u",27);
try
    { $req = $db_bbn->query($bbnq); }
catch (Exception $e)
    { bbnf_pdo_error($e,__FILE__,__LINE__); }
if ( $r = $req->fetch() )
    { echo "This query has a row result"; }
else
    { echo "This query has an empty result"; }
?>