php-lib questions

I have a php script which is like a modified shopping cart. It starts with a search that calls a perl script ( I know, but it's the only way I can include certain proprietary functionality I need). This returns a results set of documents that can be either downloaded or requested depending on permissions. If you click to request a document a php script request.php is called it is sort of like a cart script, and uses phplib sessions and db_oci8. The strange problem I am having is this: If it is the first time you have reqeusted this script since last opening your browser, you will get a 404, if you hit back/forward or refresh the script works fine as if nothing ever happened. You will not get this error again as long as you do not close and reopen your browser. Any ideas?