What is the significance of using the correct file extension, such as ".php," when including PHP code within HTML files?

Using the correct file extension, such as ".php," when including PHP code within HTML files is crucial because it tells the server to interpret the file as containing PHP code. Without the proper file extension, the server will not recognize the PHP code and will not execute it, resulting in the PHP code being displayed as plain text in the browser.

<?php
// PHP code here
?>