include
include_once
Control Structures
PHP Manual

## 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. See the documentation for require() for more information on how this statement works.

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 #1 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 access of remote files via this function, even if allow_url_fopen is enabled.

See also require(), include(), include_once(), get_required_files(), get_included_files(), readfile(), and virtual().

include
include_once
Control Structures
PHP Manual