How to Use include and require in PHP: Syntax and Examples

In PHP, include and require are used to insert the contents of one PHP file into another. include gives a warning if the file is missing but continues running, while require causes a fatal error and stops the script if the file is missing.

Syntax

include and require are followed by the file path in quotes. You can use parentheses or not. Both accept relative or absolute paths.

• include 'file.php'; - Inserts the file and continues if missing.
• require 'file.php'; - Inserts the file and stops if missing.
• Use _once variants to include files only once to avoid duplicates.

php

<?php
include 'file.php';
require 'file.php';
include_once 'file.php';
require_once 'file.php';
?>

Example

This example shows how include and require work by including a file that defines a function and then calling it.

php

<?php
// contents of greetings.php
// <?php
// function sayHello() {
//     echo "Hello, world!\n";
// }

include 'greetings.php';
sayHello();

require 'greetings.php';
sayHello();
?>

Output

Hello, world!
Hello, world!

Common Pitfalls

Missing file: include only warns and continues, which can cause errors later if the file is essential. require stops the script immediately, which is safer for critical files.

Multiple inclusions: Including the same file multiple times can cause function redefinition errors. Use include_once or require_once to prevent this.

php

<?php
// Wrong: including same file twice
include 'greetings.php';
include 'greetings.php'; // causes error if functions repeat

// Right: include once
include_once 'greetings.php';
include_once 'greetings.php'; // no error
?>

Quick Reference

Function	Behavior on Missing File	Multiple Inclusion Protection
include	Warning, script continues	No
require	Fatal error, script stops	No
include_once	Warning, script continues	Yes
require_once	Fatal error, script stops	Yes

Key Takeaways

Use require for files essential to your script to stop execution if missing.

Use include when the file is optional and you want the script to continue if missing.

Prevent errors from multiple inclusions by using include_once or require_once.

Both include and require accept relative or absolute file paths.

Always check file paths carefully to avoid warnings or fatal errors.