1PHP scripts run relative to the current path (result of getcwd()),
2not to the path of their own file.
3Using __DIR__ forces the include to happen relative to their own path.
4CORRECT: in file2.php: include (__DIR__ . "/file3.php");
5To demonstrate:
6- file1.php
7- dir/
8 - file2.php
9 - file3.php
10If file2.php includes file3.php like this: include `file3.php`.
11It will work fine if you call file2.php directly.
12However, if file1.php includes file2.php,
13the current directory (getcwd()), will be wrong for file2.php,
14so file3.php cannot be included.