PHP Include Require Reuse PHP Files: Tutorial, Examples, FAQs & Interview Tips

PHP allows you to insert the content of one PHP file into another using include and require. This promotes code reuse and keeps your project organized.

include vs require

The key difference: if the file is not found, include produces a warning and continues execution, while require produces a fatal error and stops execution.

include and require

<?php
// include - warning if file missing, script continues
include 'header.php';
include 'nav.php';

// require - fatal error if file missing, script stops
require 'config.php';   // critical - must exist
require 'database.php'; // critical - must exist

// include_once / require_once - prevent duplicate inclusion
include_once 'functions.php';
require_once 'classes/User.php';

echo "Page content here";

include 'footer.php';
?>

Practical Example - Shared Header & Footer

A common use case is splitting a page into reusable parts: header, navigation, content, and footer.

Shared Layout Files

<!-- header.php -->
<?php
$siteName = "My Website";
?>
<!DOCTYPE html>
<html>
<head>
    <title><?php echo $siteName; ?></title>
</head>
<body>
<header><h1><?php echo $siteName; ?></h1></header>

Using Shared Files

<?php
require_once 'includes/header.php';
require_once 'includes/nav.php';
?>

<main>
    <h2>Welcome to the Home Page</h2>
    <p>This is the main content area.</p>
</main>

<?php
require_once 'includes/footer.php';
?>

<!-- config.php - loaded once, shared across all pages -->
<?php
// config.php
define('DB_HOST', 'localhost');
define('DB_NAME', 'myapp');
define('DB_USER', 'root');
define('DB_PASS', 'secret');
define('BASE_URL', 'https://example.com/');
?>

Key Takeaways

include() generates a warning if the file is not found; require() generates a fatal error.
include_once() and require_once() prevent the same file from being included multiple times.
Use require_once() for critical files like database connections and configuration.
PHP resolves relative paths from the current working directory, not the script's directory - use __DIR__ for reliable paths.
Autoloading (spl_autoload_register or Composer) is the modern alternative to manual includes.
Never include files based on user input without strict validation - it can lead to remote file inclusion (RFI) attacks.

PHP Include Require Reuse PHP Files: Tutorial, Examples, FAQs & Interview Tips

include vs require

Practical Example - Shared Header & Footer

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