undefined variable

Hello, I am setting up a basic shopping cart.

Upon (form) submission of an "add to cart" button I get an undefined variable error.

This is because the "add to cart" form is submitting to "thispage" i.e itself, which contains an invoked database search function.
That search function relies on an externally provided keyword which is passed from another webpage upon the first load of that page.
How can I make that externally submitted variable exist for the duration of multiple reloads of that page ie. each time the "add to cart" buttons are submitted ?

thanks kindly for any help.