Description:
------------
Hi,
PHP's built-in webserver crashes if it's started with a router script with relative path and in a shutdown function there is a chdir to a different directory than one where the router script is.

Test script:
---------------
Create the file /home/user/router.php with the following content

<?php

function shutdownWithChdir() {
    chdir('/tmp');
}
register_shutdown_function('shutdownWithChdir');
echo 'Hello World';

?>

Now in your terminal:
$ cd /home/user
$ php -S localhost:3000 router.php

Now perform the first request to the webserver and see that correctly returns "Hello World". Then perform a second request.

Expected result:
----------------
You would expect to get "Hello World" even for the second request.

Actual result:
--------------
Instead you get:

Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
Fatal error: Unknown: Failed opening required 'router.php' (include_path='.:') in Unknown on line 0 

For every subsequent request. If you restart the webserver you'll get the same result: first request ok, subsequent requests error.

This happens only if the chdir is done inside a "shutdown function" not if it's done in the "normal" script execution.

I can confirm this bug on PHP 5.6, 7.0 and 7.1 (I do not remember the exact versions numbers) and I don't think it's related to the #74496.