We have recently run into a few issues where users were attempting to parse PHP code in .html files. We don’t recommend this, and this post is meant to explain why.
First of all, an understanding of file extensions need to be made. Technically speaking, a file extension doesn’t really mean anything. In Windows, a file extension tends to have more meaning than in other operating systems, like Linux (which is what our servers run on). File extensions are largely hidden in new versions of Windows, but people who are familiar with older versions of Windows (and DOS) may remember that files with a .exe extension were able to be “run”. These were “executable” files that would start a program or application. Likewise a .doc file would open up the file in Microsoft Word. A .txt file would open the file up in notepad.
But technically speaking, you could just as easily open up Notepad, type up some notes, and save that file as “notes.exe”. All this would really do is confuse other users because they would expect “notes.exe” to run something and in order to open and read the file, users would have to know to open the file directly from Notepad.
In this sense, we can see that a file extension’s main purpose is used to categorize files. We have been brought up in a culture where .exe file extensions are meant to be “executable”, .txt files are meant to be simple text files that can be read by Notepad, .jpg files are image files meant to be opened by an image viewer. The extension of the file is meant to serve as a way to quickly and easily categorize what type of file this is.
You can think of this also as a filing cabinet. You may have an insurance statement that you would most likely file in the Insurance folder. A bank statement would be filed in the folder label Bank Statements. Can you put the insurance statement in the Bank Statement folder? Sure, nothing will stop you. But when you go to look for that Insurance statement 2 years later, where are you going to looK? In the Insurance folder or the Bank Statement folder? A statement that is misfiled can lead to a lot of headaches later down the road.
The same is also true about putting PHP code in .html or .htm files. This just isn’t the expected behavior. PHP code has to interface with the back-end webserver. How this is done gets technical very quickly, but just know that the web server has to talk to a system on the server that understands PHP code and interprets this back to something the web server can understand. By default the web server only sends code from .php files to this system to be interpreted. While it is true that the server can be made to send .html or .htm files, it is just not the default behavior. So any change in the way PHP operates or any change to this webserver -> PHP interface system, non-standard setups (like PHP code in .html files) may get overlooked.
The TL;DR version of this is to always be sure to use file extension properly. This is going to save you a lot of headaches later on, if you follow the expected behavior for any file extension. If you want to use PHP code, you should always use a .php file extension for that file. If you choose to operate outside of this expected standard behavior, do not be surprised when you encounter rendering issues.