I would try Enable diagnostics logging for apps in Azure App Service to better determine what the issue is. If you a simple HTML file works but your PHP file doesn't, it may be an issue with the script. Just curious if a plain Jane PHP file worked e.g.
<?php
echo "This is a test";
If you're running your PHP app on Linux, try accessing the docker logs via https://<appname>.scm.azurewebsites.net/api/logs/docker
to determine if there were any PHP errors during image retrieval and app startup. PHP on Linux plan uses PHP 8 so there may be an incompatibly there; highly unlikely but without additional logging information not outside the realm of possibility.