Running custom files in wordpress enviorment

Hello

I’ve been struggling with this question for a while. To easily explain my question this is what i want to do:

<?php
$file = file_get_contents (‘wp-content/themes/mytheme/custom.php’);
Echo $file;
?>

I want to add this line into “some place” for example into page.php file. This works fine for just text, but if i want to make php work wordpress inside that file i need to add something… But add what? I understand that example for page.php its…

<?php
/*
Template Name: Something
*/
get_header(); ?>

…which binds the php with wordpress. But if i want just a random file? Is it even possible?

Regards,
Karl