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

For include its possible to set some folders where PHP search automatically. When you include a file with a relative path you search in all of that folders. Its better to define the real path to prevent some errors in loading wrong files.

https://secure.php.net/manual/en/function.set-include-path.php

Then you can be sure that you load the correct file.