make sure that the cart require_once statement is before the getContent call, for example:
<?php
//WA eCart Include
require_once("WA_eCart/WA_Store_Cart_PHP.php");
?>
<?php
$WA_Store_Cart->GetContent();
?>
if the $WA_Store_Cart->GetContent(); command where before the cart require_once command, you would get the error you are receiving.