Append php file to footer

This might help you.

add_action('wp_footer', 'insert_my_js_to_footer');
function insert_my_js_to_footer(){
?>
  <script>
    // your script here.
  </script>
<?php
};

UPDATE:

And if you want to append the php file itself then you can try this.

add_action('wp_footer', 'insert_php_file_to_footer');
function insert_php_file_to_footer(){
    include( 'your-php-file.php' );
};