行为和正确使用PHP include语句

Question

I have the following directories and files:

└── project
    ├── index.php
    └── lib
        ├── file1.php
        └── file2.php


<?php
//index.php
include 'lib/file1.php';


<?php
//file1.php
include 'lib/file2.php';


<?php
//file2.php
echo 'this is from an echo statement in file2.php';

This does work when I try it in the browser as I expected. In particular I think the include statement in file1.php makes sense because it uses a path to file2.php relative to the location of index.php which includes file1.php and so is the location that file1.php's code will be executed.

However, I was surprised that if I change the include statement in file1.php to:

include 'file2.php';

it still works.

I would like to understand why both include statements work and get an idea of which of the two is the more correct.

Answer 1

From the documentation (emphasis mine):

If the file isn't found in the include_path, include will finally check in the calling script's own directory and the current working directory before failing.

file1.php's own directory is lib, and it can find file2.php there.