php - If-statement string strange comparaison -

i've miss realy don't understand :

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) ""  

should using $cat->slug inside if statement rather $cat->$slug