Answers for "what is __dir__ in php"

how to echo line number in php

echo  __LINE__;

include php __DIR__

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.
CORRECT: in file2.php: include (__DIR__ . "/file3.php");
To demonstrate:
- 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.

__dir__ in php

<?php
include __DIR__ . "/worker_2.php";