PHP PDO：从查询中获取第一列

Question

I have a pretty stupid question, but I can't get it to work immediately.
How do I load only one field of the result array of a query into a session (array) using a single PDO statement?
I commented the missing code below:

public function getPermissions($user_role_id){
    if(!isset($user_role_id) || empty($user_role_id)){ 
        $_SESSION['user']['user_permissions'] = '';
    }else{
        $db = Database::get_database();
        $query = "
            SELECT 
                rp.role_id, rp.permission_id 
            FROM 
                role_permission_tbl rp 
            WHERE
                rp.role_id = :role_id"; 

            $query_params = array( 
                ':role_id' => $user_role_id
            );

            try 
            {
                $stmt = $db->prepare($query); 
                $result = $stmt->execute($query_params); 
            } 
            catch(PDOException $ex) 
            { 
                 die("Failed to run query: " . $ex->getMessage());
            }

            $row = $stmt->fetchAll();
            if($row){
                //I only want to retrieve the field "permission_id" 
                $_SESSION['user']['user_permissions'] = $row;
            }else{
                $_SESSION['user']['user_permissions'] = '';
            }
        }
}

Thanks

Answer 1

After seeing your later comments, it looks as though you're wanting to save all permission data in a session variable so that you can look it up by permission ID:

$rows = $stmt->fetchAll();
foreach($rows as $row){
    //Add to session, keyed by permission ID.
    $_SESSION['user']['user_permissions'][$row['permission_id']] = $row;
}

//Then, if you want to see if said permission ID #21 exists:
if(isset($_SESSION['user']['user_permissions'][21])){
    echo 'This user has permissions with ID 21!';
    $permissionDetails = $_SESSION['user']['user_permissions'][21];
    var_dump($permissionDetails);
}

Answer 2

Can you try $row = $stmt-> fetch(); instead of $row = $stmt->fetchAll(); if it is fetch only one record from table,

 $row["permission_id"];

Answer 3

Like any other "get it to work immediately" this question has contradicting conditions.
Like any other PHP code, it is ten times long than needed.
Like many other SO questions, it can be solved by quick manual lookup.

In case you need your permissions in array

public function getPermissions($user_role_id){
    $sql = "SELECT permission_id FROM role_permission_tbl WHERE role_id = ?"; 
    $stm = Database::get_database()->prepare($sql);
    return $stm->execute(array($user_role_id))->fetchAll(PDO::FETCH_COLUMN);
}

note that assigning variables inside functions is a very bad practice. So, better call it this way

$_SESSION['user']['user_permissions'] = $user->getPermissions($user_role_id);

• isset() is useless in conjunction with empty() as latter covers the former.
• both isset() and empty() are useless for the function variable too, as it is always set by design
• a verification for this particular input variable can be done, but for the sanely designed application it would be unnecessary.
• setting a variable you are going to test with in_array() to an empty string will produce an error.
• there is no use for the alias with single table.
• PDO methods can be called dramatically shorter way, there is no use for stretching one simple query call to a whole screen of code.
• echoing a system error message to a site user is an awful practice.
• the very manual page for the fetchAll() contains an exact example for this very question of getting single column out of the query result.
• there is no use for testing returned value explicitly, as it already contains either result or empty value (and luckily, fetchAll() will return even empty value of desired type).