If you're seeing this error, then you're using an old version of Zen Cart that's not compatible with the PHP version your server is running.
Specifically, Zen Cart versions 1.3.8a and older are NOT compatible with PHP 5.3.0 or newer.
SOLUTION: Upgrade your site ASAP to the latest version of Zen Cart.
IMPORTANT ALERT: ZC Versions older than v1.3.9 have some serious security vulnerabilities. You should be upgrading IMMEDIATELY!