In case some of you are wondering why zen_id is all of a sudden being added to your links (or always has) and you want it gone:
Be sure that your “Session Directory” in Configuration>Sessions is set to a directory that actually exists and is writable. If the directory doesn’t exist, then Zen Cart will starting adding zenid to URLS.
Hope this helps.

意思是说在zen-cart后台的Configuration>Sessions里有一项Session Directory,它是指定session所使用的文件夹的，如果这个文件夹不存在，或者不可写的话。那么就会在URL上显示zenid=，当然，解决方法就是让这个项所指向的文件夹存在并且可写。

转载自清水的博客