In the text/image link to shopping cart, add extra information about the current page, for example xyz.com?ref=abc. The information in ref can then retrive in the shopping cart so that sending user back to the previous page (eg. abc.php). Do not use $_SERVER['HTTP_REFERER'] as not all browser support it. if you are using form, then use hidden field to pass ref information to the shopping cart (I use this on my clients' store).

Passing the page URL using a hidden field is the safest way to do it.

You could use $_SERVER['HTTP_REFERER'], but as zippee mentioned, not all browsers pass the info. You could just simple test it for a value. If a value exists, display the Continue Shopping button with the link. If it doesn't exist, don't display the button.

My main business is SEO. IMHO, you shouldn't pass values in the URL like that if you are using hyperlinks. It creates multiple URLs that all represent the same page, which is guaranteed to cause problems with duplicate content penalties with the search engines. You could overcome those issues by either adding the proper meta tags to the cart page to prevent the indexing and archiving of the page, or by adding the rel="nofollow" attribute to the hyperlink, or by using a form button to link to the cart.