So i was adding a .php file to my github website with an email sender form, but when i try running it on my website it makes me download it instead of making it run directly in the browser. I was also trying to research something and i saw something about apache, but i don't know how to do it and how it works.
What i tried is to change the extension to .html but unfortunately, it sended me a 405 error "not allowed". I'm sure the code is right and the only thing is that it dosn't make me run it.
What i want is that the php file, runs on the browser instead of making me download it.
Assuming that by GitHub Website you mean GitHub Pages...
You can't.
GitHub websites are mostly static - you, as a developer, push html or markdown and github provides/generates static content for viewers/browsers.
If you want to have a PHP-based backend, you need to use a different service. Alternatively, if your PHP code can generate static html, you can run it in a github action and push the html into github pages - but once done, it will stay as is until the next run.
Edit: also check the video on their site, which among other things, says:
There are no databases to set up or servers to configure
You can however rebuild PHP into WASM and run your code in it, but it is relatively complicated to set up. 3v4l.org, for example, has a "live preview" mode (se) that does exactly that.