Control structures in Php: require_once()

The require_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the require() statement, with the only difference being that if the code from a file has already been included, it will not be included again.
require_once() should be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.
For examples on using require_once() and include_once(), look at the » PEAR code included in the latest PHP source code distributions.
Return values are the same as with include(). If the file was already included, this function returns TRUE

Note: require_once() was added in PHP 4.0.1

Note: Be aware, that the behaviour of require_once() and include_once() may not be what you expect on a non case sensitive operating system (such as Windows).

Example: require_once() is case insensitive on Windows

<?php
require_once "a.php"; // this will include a.php
require_once "A.php"; // this will include a.php again on Windows! (PHP 4 only)
?>

This behaviour changed in PHP 5 - the path is normalized first so that C:\PROGRA~1\A.php is realized the same as C:\Program Files\a.php and the file is required just once.

Warning:

Windows versions of PHP prior to PHP 4.3.0 do not support accessing remote files via this function, even if allow_url_fopen is enabled.