When upgrading your server from old PHP release to 5.4 make sure your application does not use deprecated functions such as session_register.
This function may be used in older CMS versions, as Tiger CMS.

To comment out all occurrences of incompatible functions in the code you can use following command:
Example for session_register:

[code]
find . -type f -exec sed -i ‘s/session_register/\/\/session_register/g’ {} \;
[/code]

Generic troubleshooting steps for error 500 in Apache:

1. Check Apache error log
/var/log/apache2/error.log

2. Turn on PHP Error reporting in etc/php.ini (/etc/php5/apache2/php.ini)
[code]
error_reporting(E_ALL);
[/code]

3. Create an info.php file to test php, with content:

[code]

[/code]

So you can see what version you use and if it is loaded correctly.

Good Luck!