500 Errors And Service Unavailable Errors

So I have V4.3.4 installed on two separate domains. Both are behaving oddly after I upgraded MySQL to version 5.6 and PHP to version 5.4.

First domain gave me a flat out 500 server error. After working with my host company, there was an .htaccess file in the root of the store showing this code:

# Use PHP53 as default

# AddHandler application/x-httpd-php53 .php

suPHP_ConfigPath /opt/php53/lib

# AddHandler application/x-httpd-php53 .php

suPHP_ConfigPath /opt/php53/lib

After commenting out all the lines in that code and clearing the cache, the website started working.

The second website with the exact same install didn't give me a 500 server, but did give me the Service Unavailable sign and message on the front end. However, I was still able to log into the admin side.

The error I was getting on the front end was:

PHP Fatal Error Message Call to a member function open() on a non-object Error at app/functions/fn.images.php, line: 633 Backtrace

I tried clearing the cache, but to no avail. I was finally able to get the website up and running by adding the following to the .htaccess file:

AddHandler application/x-httpd-php53 .php

Essentially forcing it to run on version 5.3

Sorry for the long winded post, but, can ANYBODY tell me why with 2 cscart installs of the same version are having different issues and are required to run on different versions of PHP? And ideally how to fix this issue?

Thanks in advance for any assistance.