I'm running a different version of IIS, but I have the same issue.
I spent a few hours trying to get this to work the other day and no real luck.
I don't think that the problem is osTicket however.
I manually installed php 8.0.21 VS16 NTS on my IIS 10 server and added the Handler mapper to point the site to the new version of PHP. Restarted the Site and IIS . Site displays a 500 error, and no php error log. I installed Microsoft Visual C++ 2015-2022 x64 Redistributable thinking okay new version of PHP need a newer version for it to work. Restarted the Site and IIS . Same issue. [note: yes I upgraded my osTicket files on the test site also].
I created a phptest.php and put <?php phpinfo(); ?> in it and go to it and get a PHP configuration screen.
Rechecked my Handler Mappings. killed the App Pool and made a new one. Edited the site to use the new one. Restarted the Site and IIS. Same issue.
Decided okay fine, and use WPI to install PHP 8.0.0. Checked the Handler Mappings. Restarted the Site and IIS. Same issue.
Needless to say if you get any brilliant ideas please let me know. 🙂