Control Structures
PHP Manual


(PHP 4, PHP 5)

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 may be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, so in this case it may help avoid problems such as function redefinitions, variable value reassignments, etc.

See the include documentation for information about how this function works.


With PHP 4, _once functionality differs with case-insensitive operating systems (like Windows) so for example:

Example #1 include_once with a case insensitive OS in PHP 4

include_once "a.php"// this will include a.php
include_once "A.php"// this will include a.php again! (PHP 4 only)

This behaviour changed in PHP 5, so for example with Windows 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.

Control Structures
PHP Manual