php kundorg session problem adding and removing

<?php
     $product_id = $_GET[id];     //the product id from the URL
     $action     = $_GET[action]; //the action from the URL 
	 
	  switch($action) {    //decide what to do    
    
         case "add":
			if (array_key_exists($product_id,$_SESSION['cart'])){
				echo "This item has allready been added";
			break;
			}
			else{
			$_SESSION['cart'][] = $product_id; //add $product_id to session ['cart'].
			 break;
			}
		case "remove": 
				unset($_SESSION['cart'][$product_id]); //unset the value 
			break;		

		 case "empty":
             unset($_SESSION['cart']); //unset the whole cart, i.e. empty the cart.
         break;
		
			
		 
		
	}

?>

 if (array_key_exists($product_id,$_SESSION['cart']))

<a href=\"$_SERVER[PHP_SELF]?action=remove&id=$product_id\">Remove</a></td>";

case "remove": 
				unset($_SESSION['cart'][$product_id]); //unset the value 
			break;