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
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
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).
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.
Example 16-12. require_once() is case insensitive on Windows
require_once("a.php"); // this will include a.php
require_once("A.php"); // this will include a.php again on Windows! (PHP 4 only)
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(),
get_included_files(), readfile(), and