When including PHP files in an existing webpage, is it necessary to modify the header for XHTML or HTML validation, or is it acceptable to include the entire HTML structure in the included file?

When including PHP files in an existing webpage, it is not necessary to modify the header for XHTML or HTML validation if the included file contains the entire HTML structure. It is acceptable to include the entire HTML structure in the included file as long as it is valid HTML. This approach can help keep your code organized and make it easier to manage and update in the future.

<?php include 'header.php'; ?>
<!-- Content specific to this page goes here -->
<?php include 'footer.php'; ?>