After 8.5 years my homeserver finally has a (password protected) registration page. As a fail-safe it sends me a message whenever a new user is registered.
Also wanted to try interweaving and directly instead of using one long <?php ?> block with echoes.
The end result is a little bit messy in some places, but it was fun.
Source code:
https://gitea.lubiland.de/lubiland/register-imninja/src/branch/main/index.php