How to debug vars inside function at functions.php file?

You can simply use var_dump() to do this. That is how I check values inside functions and filters.

I have the following line of code in a file which I simply copy and paste where needed to dump the value of a variable

?><pre><?php var_dump( $variable_to_test ); ?></pre><?php

The pre tags dump a nice readable array/object/string depending on the value. Inside your function you can just do

?><pre><?php var_dump($ad_posts); ?></pre><?php 

after ksort( $ad_posts );.

Just make sure to call the function somewhere if it is not hooked to some kind of hook otherwise nothing will happen