if not $_POST,not $_GET not $_REQUEST...what is it?

Hi, I am getting some values from 2checkout...I wanted to know in what form so I made this simple little test script... <? if($_POST[$product_id] >=3 && $_POST[$product_id] <=8) {print("post is true");} elseif($_GET[$product_id] >=3 && $_GET[$product_id] <=8) {print("GET is true");} elseif($_REQUEST[$product_id] >=3 && $_REQUEST[$product_id] <=8) {print("request is true");} else {echo 'None worked';} ?> The $product_id that I am passing is 6 but I always get the message "None worked"....What should I do? what else is there? Kindly reply, -Ryan