PHP include vs require vs require_once: what is the difference?

include and require both load and run another PHP file; they differ only when the file cannot be loaded. include emits a warning and the script continues, while require stops with a fatal error. The _once variants (include_once, require_once) do the same but skip the file if it was already loaded, which avoids "Cannot redeclare function" errors. Use require_once for files the program cannot run without (configuration, function libraries), include for optional parts such as templates, and Composer's autoloader for classes.

Differences

Example

<?php
require_once __DIR__ . '/config.php';     // must exist: stop if it is missing
require_once __DIR__ . '/vendor/autoload.php';

$settings = include __DIR__ . '/settings.local.php'; // a file can return a value
if ($settings === false) {
    $settings = [];                        // optional file: continue with defaults
}

include __DIR__ . '/templates/footer.php'; // optional part of the page

When the file is missing, include reports Warning: include(settings.local.php): Failed to open stream: No such file or directory and goes on; require reports the same warning followed by a fatal error, Failed opening required ..., and stops.

Notes

• They are language constructs, not functions: require 'file.php'; and require('file.php'); are the same.
• A bare file name ('lib.php') is searched in include_path, then in the calling script's directory and the current working directory; a path starting with ./ or ../ is relative to the current working directory only. Use __DIR__ . '/file.php' to make paths independent of where the script is started.
• For classes, rely on autoloading (spl_autoload_register, or Composer's vendor/autoload.php) instead of one require_once per class.