Some important info below from zen cart development team!

PHP 5.3 was released a few months ago, and along with it come many problems due to the PHP authors’ decisions to backport many PHP 6 features into 5.3 and deprecate several oft-required functions.

In so doing, they broke many things affecting Zen Cart operation in v1.3.8a and older.

We are preparing a patch which can be used by those whose hosts have upgraded (or have announced imminent plans to upgrade) to PHP 5.3.

We hope to post the patch here very soon. It’s in regression testing now.

When the patch is ready it will be announced here in the Announcements area.

via PHP 5.3 Support – Zen Cart Support.