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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
require_once() was added in PHP 4.0.1
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 7.11. 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.
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.
See also require(), include(), include_once(), get_required_files(), get_included_files(), readfile(), and virtual().