PHP Manual
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include_once()

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

Note: include_once() was added in PHP 4.0.1pl2

Note: 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).
Example 16-13. include_once() is case insensitive on Windows
<?php include_once("a.php"); // this will include a.php include_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 included 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.