Hi
In case of PHP7.4 or less, it can be controlled by htaccess as brtrach says.
If you have PHP 8.0, it will run on Nginx instead of Apache, so you need to prepare a startup script to modify the config.
I've verified something similar before and put it on my blog.
https://dev.classmethod.jp/articles/app-service-linux-php-default-document/
I hope it helps you.