What is the difference between require, require_once and include in PHP?

In PHP, require, require_once, and include are all functions that can be used to include a file in a PHP script. The main difference between these functions is how they handle multiple calls to the same file.

• require: This function includes and evaluates a specified PHP file. If an error occurs during the inclusion, a fatal error is raised and the script execution is halted. require can be called multiple times, and the file will be included and evaluated each time.
• require_once: This function works the same as require, but it checks if the file has already been included before calling it. If the file has already been included, require_once will not include it again, and the code in the file will not be evaluated again. This can be useful to prevent multiple declarations of functions or variables, which can cause problems in your code.
• include: This function works the same as require, but it generates a warning instead of a fatal error if an error occurs during the inclusion. This means that the script execution will not be halted, and the code after the include statement will be executed. Like require, include can be called multiple times, and the file will be included and evaluated each time.

Here is an example of how these functions can be used in a PHP script:

<?php

// Include and evaluate a file
require 'file1.php';

// Include and evaluate a file, but only once
require_once 'file2.php';

// Include a file and generate a warning if an error occurs
include 'file3.php';