PHP Include and Require

As pages grow, you don’t want to copy-paste the same header, footer, sidebar, or database connection code into every file. PHP lets you pull code from another file into the current one using include, require, include_once, and require_once. This page explains the differences and shows two real files being included live right here on the page.

1) include vs require

Both statements pull the contents of one PHP file into another at the point they are called. The difference is what happens when the target file is missing.

<?php
  include 'no-such-file.php';
  echo 'This line still runs.';
?>

<?php
  require 'no-such-file.php';
  echo 'This line never runs.';
?>

2) include_once and require_once

If the same file gets included twice (for example, two different includes both need a config file), PHP will happily run it twice — which can cause “function already declared” or “class already declared” fatal errors. The _once variants track which files have already been loaded and skip them the second time.

<?php
  include_once 'config.php';
  include_once 'config.php'// safe, loaded only once
?>

3) Typical Use Case: Shared Header/Footer Files

The most common reason to use includes is to avoid repeating the same markup on every page — navigation bars, sidebars, footers. You are looking at a live example of this pattern right now: every tutorial page on this site, including this one, is built from small reusable pieces.

<?php include_once('php-header.php'); ?>
<div class="wrapper">
  <?php include_once('php-sidebar.php'); ?>
  <!-- page content goes here -->
</div>

4) Path Resolution Gotchas

A relative path like 'includes/config.php' is resolved relative to the currently executing script, not necessarily the file that wrote the include line. If file A includes file B, and file B includes a relative path, PHP still resolves it based on the entry script’s working directory — which can silently break when the same file is included from pages that live in different folders.

<?php
  // Fragile: depends on which script is currently running
  include 'connection.php';

  // Robust: always resolved relative to THIS file's own folder
  include __DIR__ . '/connection.php';
?>

5) Live Demo: Real Includes Running on This Page

The two boxes below are not a screenshot or a copy-pasted example — they are produced by two tiny separate files, tryit/inc-demo-header.php and tryit/inc-demo-footer.php, pulled into this exact page right now using real PHP include statements:

<?php
  include __DIR__ . '/tryit/inc-demo-header.php';
?>

<?php
  include __DIR__ . '/tryit/inc-demo-footer.php';
?>

6) Best Practices & Common Mistakes