One of my clients asked me to debug his site, because when php5 is enabled from the control panel the site does not show anything, just white pages!

The site was built using Joomla! 1.5, so I enabled debugging mode and set the error reporting level to maximum to know what is the problem.

Then this error was displayed when opening any page:

Parse error: syntax error, unexpected $end in filename:lineno

it was the index.php file of the template, and after some debugging I found that the problem is the using of short php tags, like this:
<?
The server was not configured to allow short tags.
For maximum compatibility, it is preferred to use full tags, like this:
<?php
Hope this help you all!