PHP Won't Load Extension on IIS10

Installed:
Windows 2019 Server with IIS10
PHP 5.3 (needed for old PHP application)

Issue:
After configuring ANY extension in php.ini , no PHP files can be loaded.
If I remove the extension from php.ini , php files load ok.

I tried:
php.ini correct path
extensions correct path
PHP in system variables

I cannot provide access to the server, need basic guidance.
If you're a professional, it shouldn't take more than 10 minutes.

Any reply without " I understand the issue and I can explain how to fix it without server access " will be ignored.