Website not working - after change address

I build android app that communicate with PHP and it was working fine until I changed my web address.
The problem is if I add includefunction.php`; to the PHP file it stop working.
please someone can help me

The new address of your website is ntwaysolutions.000webhostapp.com

Please check and make sure that all addresses from your PHP scripts are matching the new one.

I know this is the address but when I try to add the php file -> https://ntwaysolutions.000webhostapp.com/test.php . it not working.

test.php:

<?php include `test1.php`; $a = 1; $b = 2; $res = sum($a, $b); echo "Hi ". $res; ?>

test1.php"

<?php function sum($a, $b) { return $a + $b; } ?>

if i remove ( $res = sum($a, $b); ) from test.php it working

please help

There is no such function as sum() in PHP.

Use $a + $b instead.

I know that is no such function.
I create it.
The problem is that I can’t call function from other file or to write include myFile.php.

include_once("myfile.php");

…or…

require_once("myfile.php");