PHP Include / Require

PHP has four ways to bring code from another file into the current one: include, require, include_once, require_once. The difference comes down to "warning vs error" and "load once vs every time".

The matrix

Use __DIR__ for paths

Relative paths depend on the script that started PHP — fragile. Always pin paths to the file you're in:

require __DIR__ . '/lib/config.php';

What gets included

The included file runs in the current scope. Variables defined inside it become available, functions and classes get registered globally. Most teams keep included files free of side effects — just declarations.

Modern alternative: Composer autoload

For class files, don't write require at all. Composer generates an autoloader that finds classes by name:

require __DIR__ . '/vendor/autoload.php';

use App\Billing\Invoice;
$inv = new Invoice();    // file auto-loaded by Composer

When to use which

• require — the file is essential. App can't run without it.
• require_once — library files (functions, classes) you don't want to load twice.
• include — optional templates or fragments; missing is OK.
• include_once — optional templates you don't want duplicated.

Example

<?php
// header.php:  <h1>My site</h1>
// include 'header.php';      // warns if missing
// require 'header.php';      // errors if missing
// include_once / require_once for libraries.
echo 'use require for code you can\'t run without';

Exercise

Test yourself