The include_once() statement includes and evaluates
the specified file during the execution of the script.
This is a behavior similar to the include() statement,
with the only difference being that if the code from a file has already
been included, it will not be included again. As the name suggests,
it will be included just once.
include_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 more 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
include_once() was added in PHP 4.0.1
Be aware, that the behaviour of include_once()
and require_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 included just once.
Example #1 include_once() is case insensitive on Windows
include_once "a.php"; // this will include a.php
include_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 access of remote files via this function,
even if allow_url_fopen is enabled.
See also include(),