Php 5

[gjh42]

Do you have any mods on the misbehaving site aside from the template? Are they the exact same mods that you have on the site that is working correctly (same version, settings...)?

[lat9]

It's so weird why when I change from PHP 5.2 to PHP 5.3 the site will not work. Really frustrated as I can't determine what is going wrong and why such a minor upgrade to PHP stops my site from working.

The change from PHP 5.2.x to 5.3.x was not "minor", see http://php.net/manual/en/migration53.changes.php. As lhungil said, Zen Cart v1.3.9/1.5.x work out-of-the-box with PHP 5.3 so it's one (or more) of your addons that's causing you the issue.

Check in your /logs directory for the most recent file named debug*.log and post its contents; that will help others help you.

[DrByte]

It could also be that your host's PHP configuration settings are very different between PHP 5.2 and 5.3, and one of those settings is conflicting with something Zen Cart is doing.
Get the actual details of the problem from the server's PHP error logs. Your hosting company can access that information for you.