If-statement string strange comparaison

I've probably miss something but I realy don't understand that :

if(trim($_GET[$slug]) == trim($cat->$slug)) {
    $selected = 'selected';
}
else {
    $selected = '';
}

var_dump(trim($_GET[$slug]));
var_dump(trim($cat->slug));
var_dump($selected);

Show :

string(8) "albanais" 
string(8) "albanais" 
string(0) ""