You need to define a constant containing an array but the code below doesn’t work ?

define('FILTER_ECOLE', array('price, categorie, niveau'));

You get an error :

Notice: Use of undefined constant FILTER_ECOLE - assumed 'FILTER_ECOLE'  in /home/www/adin/app/code/local/Adin/Catalog/Block/Nav/Catalog/Layer/View/Sidebar.php on line 50

To define an use a constant containing an array you can simply serialize it :

#define
define('FILTER_ECOLE', serialize(array('price, categorie, niveau')));

#use
$filter = unserialize(FILTER_ECOLE);