Why include __DIR__ in the require_once?

Question

For example, I always see autoloaders called like this:

require_once __DIR__ . \'/../vendor/autoload.php\';

What is the difference between

Answer 1

PHP scripts run relative to the current path (result of getcwd()), not to the path of their own file. Using __DIR__ forces the include to happen relative to their own path.

To demonstrate, create the following files (and directories):

- file1.php
- dir/
   - file2.php
   - file3.php

If file2.php includes file3.php like this:

include `file3.php`.

It will work fine if you call file2.php directly. However, if file1.php includes file2.php, the current directory (getcwd()), will be wrong for file2.php, so file3.php cannot be included.