Magento How to debug blank white screen

Question

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 checke

Answer 1

In my case size of the index.php file was zero. I copied the original file back and it worked.

However, no idea about what erased the content of index.php.