Issues when combining debug_backtrace with auto_prepend_file

Question

I have a PHP script that will allow me to track which file required it via debug_backtrace(). I wanted to auto prepend (Via auto_prepend_file within php.ini) the file so then I could build a log of the frequency in which each PHP script is ran for internal analysis.

It works fine when I create a file that requires it however it causes an error when I "auto_prepend_file" it. My understanding of auto prepend was that it will require whatever file you identify. However, the returned array of debug_backtrace appears to be empty when auto-prepending. Any ideas on what may be happening?

Code example for those curious

first.php

<?PHP
require('second.php');
?>

second.php

<?php 
var_dump(debug_backtrace());
?>

For the actual auto prepending I'm just identifying second.php. When auto prepending I get:

array(0) { }

When going to first.php within the browser I get:

array(1) { [0]=> array(3) { ["file"]=> string(39) "/var/www/first.php" ["line"]=> int(4) ["function"]=> string(7) "require" } }

Any and all help is appreciated!

Thanks.