PHP confused about use of __DIR__ and ../ in a path - php

Question

I came across a project with this structure.
-projectRoot
--src
---bootstrap.php
--composer.json
--public
---index.php
---css
---images
Now in public directory, index.php contains the following
//in projectRoot/public/index.php
<?php
require __DIR__."/../src/bootstrap.php";
?>
I understand that __DIR__ constant resolve to absolute directory of the current executing php script file.So in the above example I would expect it to be ProjectRoot/public.
the quention
Which path will require in index.php look into ? and what does a parent dir .. in the path above resolve to ?
PS: I read that the whole point of the setup above is to ensure relative paths continue to work regardless where index.php is called. How does that work exactly? thanks.

Answer 1

<?php
require __DIR__ . "/some_file.php";
/* __DIR__ SIMPLY POINTS TO THE DIRECTORY IN WHICH THE ACTIVE SCRIPT LIVES
- THE DIRECTORY OF THE EXECUTING SCRIPT... THEN REQUIRES THE FILE
"some_file.php" WITHIN THAT DIRECTORY.
THE REAL-PATH SHOULD REFLECT SOMETHING LIKE: projectRoot/public/some_file.php
*/
require __DIR__ . "/../src/bootstrap.php";
/* FOLLOWING THE LOGIC ABOVE, __DIR__ . "/../" __DIR__ AGAIN POINTS TO
THE DIRECTORY IN WHICH THE ACTIVE SCRIPT LIVES WHILE "/../" TELLS
THE REQUIRE DIRECTIVE TO LOOK ONE DIRECTORY ABOVE THE CURRENT DIRECTORY
(AND LOCATE WITHIN THAT DIRECTORY ANOTHER DIRECTORY CALLED "src" IN THIS CASE...)
THEN WITHIN THAT "scr" DIRECTORY LOOK FOR A FILE CALLED:
"bootstrap.php" AND REQUIRE/INCLUDE IT...
THE REAL-PATH SHOULD REFLECT SOMETHING LIKE: projectRoot/src/bootstrap.php
*/
// TO UNDERSTAND THESE CONCEPTS MORE CLEARLY, IT WOULD BE ADVISED TO
// TRY SOMETHING LIKE THESE:
var_dump( realpath(__DIR__ . "/../src/bootstrap.php") );
var_dump( realpath(__DIR__ . "/index.php") );

Answer 2

Here's what's happening in your index.php:
__DIR__ // This will be projectRoot/public
/../ // This will go one level up so it'll be projectRoot
src/ // This will go inside the src folder (projectRoot/src)
bootstrap.php // This will open the bootstrap.php
So in the end, it's going to target projectRoot/src/bootstrap.php... that's the file it's requiring.