If you manually installed Zen Cart, instead of using Value Applications, you need to manually upgrade as they release updated versions of the application.
Zen Cart 1.5 supports PHP 5.2 to 5.3. If you are using an older version of Zen Cart, you can use PHP 5.2 to access the site until you upgrade.
NOTE: If you are using Zen Cart schema 1.3.8a, you need to upgrade to 1.3.9 before upgrading to version 1.5. Otherwise, you will get errors.
If you have issues connecting to the admin area, or if the application won't let you stay logged in to the Admin area, refer to this Zen Cart help article.
If you run PHP 5.2 you need to disable APC and use Zend Optimizer. For more information, see Disabling APC on Your Hosting Account and Using Zend Optimizer in Web & Classic hosting.
If you run PHP 5.3 you need Zend Guard loader. For more information, see Using Zend Guard Loader in Web & Classic Hosting.