What is the convention for breaking up a long HTML file?
I am currently using <?php include("storage"); ?> and putting all the code in a file call “storage” with the extension removed so it cannot be accessed by visitors to my site. This is probably not right.
I wouldn’t say that breaking up a long HTML file is in itself conventional enough to warrant a convention for how to do it. There is no gain to the end user, the same size chunk of HTML will have to be passed in a single request.
Is this only for your convenience while developing? In that case I would prefer a build script that combined the files before deploying them, rather than having to combine them at server side for each request.
However, if the file is so big that you can’t work in it efficiently, you should probably consider if that’s something you want to send to the user at each request.
As others have pointed out, there’s nothing specific about removing file extensions that protects them from being accessed by the user, unless you explicitly configure your server to do that.