How to include files in Wordpress function file?

 To include any file in the function file of WordPress just creates a file with the name

 i.e custom.php
                   
                              custom.php

<?php


====================Your code===============

?>


Go to function.php

Use anyone.

1) include ()

 Include function includes the specified file and it will throw PHP warning if the file isn't found.

include(dirname( ABSPATH ) .'/admin-page.php');

this will return /var/www/vhosts/foo.bar/httpdocs/includes/baz.php

2) include_once() 
include_once() works as inlude() and it will not include file
 again if already included.
include_once( get_stylesheet_directory() .'/admin-page.php');

3)  require()
require() works same as include() but PHP fatal error will be trown 
if the file is not exists
require(dirname( ABSPATH ) .'/admin-page.php');
4) require_once()
require_once() performs as require() but fill will be not included 
second time.

require_once( get_stylesheet_directory() .'/admin-page.php');

For Parent Theme: get_template_directory()
or get_template_directory_uri()

For Child Theme: get_stylesheet_directory()
or get_stylesheet_directory_uri()