//**************************************************************
//index.php
if ( isset( $_POST['rel'] ) ){
	$_SESSION['cart'][] = $_POST['rel'];
}

echo '<br /><a href="index.php?rel='.$row['rel_id'].'">ADD</a></td><td>'.$row['name'];



//**************************************************************
//checkout.php
foreach ($_SESSION['cart'] as $prod){
	echo '<br />'.$prod;
}