Unusual but valid headers in requests are not available to PHP when running via CGI

Description

The following code:

<?php
print_r($_SERVER);

Requested with:

curl -s -H 'X-Test: 1' -H 'X-%%Test%%: 1' 'http://127.0.0.1/' | grep 'HTTP_'

Resulted in this output:

[HTTP_X_TEST] => 1
[HTTP_ACCEPT] => */*
[HTTP_USER_AGENT] => curl/7.83.1
[HTTP_HOST] => 127.0.0.1

But I expected this output instead:

[HTTP_X_TEST] => 1
[HTTP_X_%%TEST%%] => 1
[HTTP_ACCEPT] => */*
[HTTP_USER_AGENT] => curl/7.83.1
[HTTP_HOST] => 127.0.0.1

This appears to be unique to CGI, or alternatively mod_php is immune.

PHP Version

PHP 8.1.26

Operating System

Alpine Linux 3.18

Alpine Linux 3.18 上で CGI を介して PHP 8.1.26 に対してリクエストを再現し、CGI が $SERVER にどのように値を設定するかを調査し、その結果を mod_php と比較します。X-%%Test%% という通常とは異なるヘッダーが HTTP_X%%TEST%% として利用でき、既存のヘッダーも引き続き利用できれば完了です。