Magento How to debug blank white screen

I have an error in my Magento backend that results in a blank screen (WSOD). I have set errors to on in admin but there is nothing being created in var/logs/. (I have checked permissions for that directory and all is correct).

I have also ini_set('display_errors', 1) in index.php and Magento is set to developer mode. I have also enabled apache and php error logs.

No errors are being logged though?!

Anyone have a clue as to why errors aren't being shown? As I'm confused! Thanks

This is how I got it corrected(Hope will help you guys):

1. Use the following code in your index.php file

   ini_set('error_reporting', E_ERROR);
   register_shutdown_function("fatal_handler");
   function fatal_handler() {
       $error = error_get_last();
       echo("<pre>");
       print_r($error);
   }
   

2. In my case it tolde me that error/503.php was unavailable.

3.The issue was with testimonial extension I used(http://www.magentocommerce.com/magento-connect/magebuzz-free-testimonial.html)

4. I deleted the testimonial.xml file in my app/etc/modules/testimoanial.xml.
5. delete “maintenance.flag" file.

Whenever this happens the first thing I check is the PHP memory limit.

Magento overrides the normal error handler with it's own, but when the error is "Out of memory" that custom handler cannot run, so nothing is seen.