Warning: in_array() expects parameter 2 to be array, null given

Question

I am getting the following warning when trying to add data to a session (and checking if it already exists).

Warning: in_array() expects parameter 2 to be

Answer 1

Your $_SESSION['product'] is empty. Try this,

if(!empty($_SESSION['product']) && isset($_GET['product']) && !in_array($_GET['product'], $_SESSION['product'])){
    $_SESSION['product'][] = $_GET['product'];
}

It should work.