If you are battling with a php application using php5 which is showing no errors then your experiencing what many have. 

For some reason; most probably security, the standard setting for errors in PHP5 is "off".

 All well and good for those who don't want to see there errors but for debugging its a total nightmare but there is a line of code which can help.

 ini_set('display_errors','On');

This line of code will display all php errors which can also be a little annoying.  I like the balance of hiding the not so problematic errors with these two lines of code.

ini_set('display_errors', 'On');
error_reporting(E_ALL ^ E_NOTICE);

What your doing here is telling php to display errors, then limiting the errors to only the more important ones which seems to work for most.