As a first step, try to add the debug code to handleRequest() method of the \XLite\Controller\AController class.
You need to make sure that your request is even handled by controllers.
Please, make sure that your debug the code in the <X-Cart>/var/run folder.
Finally, you can use built-in logger instead of writing your own one:
http://kb.x-cart.com/display/XDD/Debugging+X-Cart#DebuggingX-Cart-Logging