PHP require_once

In PHP, require_once is a language construct used to include and execute a specified file in a PHP script. It is similar to the require statement but with one important difference: require_once ensures that the file is included only once in the script, even if it is called multiple times. This prevents duplicate inclusion of the same file and avoids errors that may arise from redeclaring functions or classes.

The require_once statement is commonly used to include files that contain important functions, configurations, or class definitions that are needed throughout the application.

Syntax:

require_once 'path/to/file.php';

Example:
Let’s assume we have a file named config.php that contains some configuration settings and a file named functions.php that contains custom functions.

config.php:

<?php
$config = array(
    'db_host' => 'localhost',
    'db_user' => 'username',
    'db_pass' => 'password',
    'db_name' => 'database'
);
?>

functions.php:

<?php
function add($a, $b) {
    return $a + $b;
}
?>

main.php:

<?php
// Include the config.php file
require_once 'config.php';

// Use the configuration settings
echo $config['db_host']; // Output: localhost

// Include the functions.php file
require_once 'functions.php';

// Use the add function from functions.php
$result = add(10, 5);
echo $result; // Output: 15
?>

In this example, main.php includes config.php and functions.php using require_once. If main.php is included in other scripts or files using require_once, the config.php and functions.php files will not be included again, ensuring that the configuration settings are not redefined, and the add function is not redeclared.

Using require_once helps manage file inclusion effectively in PHP applications and prevents common errors related to redeclaration of functions or variables when files are included multiple times.