How can PHP developers ensure that included files do not disrupt the flow of their web application?

PHP developers can ensure that included files do not disrupt the flow of their web application by using the `require_once` or `include_once` functions instead of `require` or `include`. This will prevent the same file from being included multiple times, avoiding conflicts and errors in the application. 

<?php
require_once 'header.php';
// Code for the web application
require_once 'footer.php';
?>

Keywords

include require include_once require_once file paths

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