Clear custom $_session variable on form submit

Hi,

Can anyone help me to clear some $_SESSION data when I submit a specific form. I have tried using this hook (gform_after_submission) but can figure this out.

EG code

add_action("gform_after_submission", "empty_session_data", 10, 2);
function empty_session_data($entry, $form){

//getting post
if($entry["post_id"] == 111) {
unset($_SESSION['interested']);
}
}

Thanks