Case mismatch in `SCRIPT_NAME` on IIS with Rewrite module

Description

I'm encountering a strange behavior on IIS regarding the $_SERVER['SCRIPT_NAME'] variable.

On a Windows 11 machine, I have an IIS running with PHP via FastCGI. I've configured a virtual directory named routingtest. in that directory, I have an index.php file that just dumps the script name and the request URI.

<?php

var_dump($_SERVER['SCRIPT_NAME']);
var_dump($_SERVER['REQUEST_URI']);

So, if I point my browser to http://localhost/routingtest/index.php/foo/bar, I see the expected output:

string(22) "/routingtest/index.php"
string(30) "/routingtest/index.php/foo/bar"

Since directory and file names are case-insensitive in Windows and IIS, I can reach the same script as http://localhost/RoutingTest/index.php/foo/bar and the two variables reflect the different case:

string(22) "/RoutingTest/index.php"
string(30) "/RoutingTest/index.php/foo/bar"

That's still as it should be. Now, I add a rewrite rule because I want to get rid of the index.php in my URLs:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="FrontController">
                    <match url=".*" />
                    <conditions trackAllCaptures="true">
                        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                        <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
                    </conditions>
                    <action type="Rewrite" url="index.php/{R:0}" />
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>

Calling http://localhost/routingtest/foo/bar:

string(22) "/routingtest/index.php"
string(20) "/routingtest/foo/bar"

Looks correct. Calling http://localhost/RoutingTest/foo/bar:

string(22) "/routingtest/index.php"
string(20) "/RoutingTest/foo/bar"

Note how SCRIPT_NAME still contains the value from the first request while REQUEST_URI is updated correctly. If I restart the IIS and make the two requests in reverse order, I get:

http://localhost/RoutingTest/foo/bar:

string(22) "/RoutingTest/index.php"
string(20) "/RoutingTest/foo/bar"

http://localhost/routingtest/foo/bar:

string(22) "/RoutingTest/index.php"
string(20) "/routingtest/foo/bar"

This means the first request after the IIS has started controls how SCRIPT_NAME is reported for all subsequent requests until the server is restarted again. This behavior confuses e.g. Symfony's HttpFoundation which uses SCRIPT_NAME and REQUEST_URI to determine the base path to the front controller script.

I don't really know if this is a bug in PHP or if this has to be fixed in IIS or the rewrite module. If I have to take this somewhere else, please point me into that direction.

PHP Version

8.1.22

Operating System

Windows 11

Start by reproducing the sequence with index.php and the IIS Rewrite configuration on Windows 11 with PHP 8.1.22 via FastCGI. Compare SCRIPT_NAME and REQUEST_URI across differently cased requests, then determine whether the behavior originates in PHP or IIS/Rewrite. Done means identifying the responsible component and documenting or implementing an appropriate resolution.