I do have an index.php file which includes different parts of the page via PHP.
For example this includes the <head> section of the HTML page into the index.php file:
<!doctype html>
<html>
<head>
<?php include($_SERVER['DOCUMENT_ROOT']."/PATH-TO-FILE/head.php"); ?>
</head>
<body>
...
</body>
</html>
The content of "head.php" may be something like this:
<meta charset="utf-8">
<title>Title of page</title>
<meta name="description" content="Description text...">
...
<link rel="stylesheet" href="stylesheet.css">
...
and so on...
Technically "head.php" is not a PHP file because it does not contain any PHP code. Neither is it a valid HTML document. It is just a HTML code fragment.
Until now I have always named these HTML code fragment files either *.html or *.php.
My question is whether this is correct or not?
Is it advisable to not give it a file extension at all? Instead of "head.php" simply "head"?
Please keep in mind that certain directives can be set in the server's .htaccess file concerning the caching and expiration of *.html and *.php files. Removing the file extension and renaming it from "head.php" to "head" may exclude it from the mentioned directives in the .htaccess file.
While searching I found this question here on StackOverflow: What extension should I use for files containing fragments of HTML
But that question was asked 6 years ago and there are so many different file extensions mentioned there, it's difficult to say which one to use.
Can anyone give an updated answer concerning this issue?