Non-existent constant return true - php

Question

Each time user logs in, I extract a serialized array containing rights. When I unserialize the array and try to assign each right to a constant, non-existent constants return TRUE. For instance, here is my call to database :
$req_level = $pdo->prepare('SELECT * FROM users_levels WHERE level_id = ?');
$req_level->execute(array($row['level_id']));
$row_level = $req_level->fetch();
$user_level_name = $row_level['name'];
$permissions = unserialize($row_level['permissions']);
foreach ($permissions as $permission) {
define(strtoupper($permission), 1);
}
$req_level->closeCursor();
My array ($permissions) is :
Array ( [0] => users_read [1] => users_update [2] => customers_read [3] => customers_update )
When I call all possible constants I should get TRUE for those existing and FALSE for those non existing, but here is what I got...
USERS_READ : <?=USERS_READ?> <br/>
USERS_UPDATE : <?=USERS_UPDATE?> <br/>
USERS_CREATE : <?=USERS_CREATE?> <br/>
USERS_DELETE : <?=USERS_DELETE?> <br/>
When I echo constants...
USERS_READ : 1
USERS_UPDATE : 1
USERS_CREATE : USERS_CREATE
USERS_DELETE : USERS_DELETE
So everything returns TRUE in this case.
Any idea ?

Answer 1

Typically, in PHP, constants that are not found are turned into strings with the same name. So, trying to use a constant 'USERS_READ' becomes the string 'USERS_READ' when attempted to be used. The string 'USERS_READ' when converted to boolean will be true.
So, you need to use the method defined in order to check if it exists before trying to evaluate the constant:
http://php.net/manual/en/function.defined.php
Or, you could do something like this:
function userHasPermission( $permission ) {
return constant($permission) === 1;
}
BTW - on a side note, using "dynamic constant" sounds like a design idea you might want to rethink. Constants are usually constant :)