The require function cannot identify and load the php file of the current folder

My question may be strange, but it really confused me!

In my child theme, I used to call and load the files next to the function.php file without any problem, but after deleting and re-inserting all the files in the child folder, it is not possible to identify the files. I used the following commands.

Inside the function.php file, I call its side file, the user_function.php file, but it is not recognized even with the command

if (file_exists('user_function.php')) {
     require_once('user_function.php');
} else {
     echo "Please try back in 30 minutes...\n";
     die;
}

The file is not recognized! and displays the error message!!!

Of course, my site is loaded, but when I go to wp-admin, it is clear that the user_function.php file is not loaded.

I have only these two files in my child theme folder:

• function.php
• user_function.php

Where is the problem from? (These instructions are not really complicated or difficult and I have used these instructions many times before in different places, but now they are challenging me)

I switched several times between other themes and even the parent template

+update:

I ran into a strange problem! Now, every other file that I want to load into function.php and user_function.php, I have to address it with __DIR__, I think the problem must be somewhere else!
I sent a hosting ticket, they say that a problem happened a few hours ago and the connection with the database was disconnected, and they did not explain what happened, but I think that this must have caused the problem and damaged my WordPress path or addresses. Is it possible that the problem is from Did this happen?

>Solution :

It may be due to the fact that the function.php is called from another file in another directory.

In order to avoid ambiguities, you can use a full path by providing __DIR__, that returns the directory of the current file, so that PHP will search for the user_function.php file in the same directory than the function.php file:

// in function.php
if (file_exists(__DIR__.'/user_function.php')) {
     require_once(__DIR__.'/user_function.php');
} else {
     echo "Please try back in 30 minutes...\n";
     die;
}