Inline PHP vs functions.php

When I put
<?php echo get_the_title($post->post_parent) ?>
into my theme template the output is correct.

However, if I try to put it into a function and then call it from functions.php it does not work. Are there some sort of global variables that I should be calling?

Should be obvious, no? You are directly using the global variable $post (you’re trying to access the $post->post_parent variable). So you need to put global $post; at the top of your function.

function whatever()
{
global $post;
echo get_the_title($post->post_parent);
...