How to use __dir__?

You can use __DIR__ to get your current script's directory. It has been in PHP only since version 5.3, and it's the same as using dirname(__FILE__). In most cases it is used to include another file from an included file.

Consider having two files in a directory called inc, which is a subfolder of our project's directory, where the index.php file lies.

project
├── inc
│   ├── file1.php
│   └── file2.php
└── index.php

If we do include "inc/file1.php"; from index.php it will work. However, from file1.php to include file2.php we must do an include relative to index.php and not from file1.php (so, include "inc/file2.php";). __DIR__ fixes this, so from file1.php we can do this:

<?php
include __DIR__ . "/file2.php";

To answer your question: to include the file warlock.php that is in your included file's upper directory, this is the best solution:

<?php
include __DIR__ . "/../warlock.php";

I've been looking to use an executed in apache's root htdocs _DIR_ variable and be able to include other scripts containing sensitive data such as database login credentials sitting outside it. I struggled a bit trying different options but the below is working really well.

Firstly, in my apache virtual host config I set/include a full linux path to apache's htdocs (you can add more paths by appending at the end :/path/to/folder/):

php_value include_path          ".:/var/www/mywebsite.ext/uat/htdocs"

Then in .htacess stored in apache's htdocs root (and git repo):

php_value auto_prepend_file     "globals.php"

Both of the above can be set in .htacess although it wouldn't work well for multiple environments suchas as DEV, UAT, PRODUCTION in particular when using git repo.

In my globals.php file inside apache's htdocs I have defined a variable called DIR that's globally used by htdocs php scripts:

define('DIR', __DIR__);

Then in each file am was now able to include/require necessary files dynamically:

require_once(DIR.'/file_folder_inside/apache's_htdocs/some-file.php');

The DIR variable would always resolve to /var/www/mywebsite.ext/uat/htdocs no matter where in the tree I call it in the above example producing

/var/www/mywebsite.ext/uat/htdocs/file_folder_inside/apache's_htdocs/some-file.php.

Now, I was looking to access a php file that's sitting outside my apache's htdocs root folder which is also easily achievable by using:

require_once(DIR.'/../apache's_htdocs_parent_folder/some-file-stored-outside-htdocs-eg-snesitive-credentials.php');