Usually this file is stored in the include_path which is set in the php.ini, however, it can be in any other path as long as known.

include_once is similar to include statement, with the only difference being that if the code from a file has already been included, it will not be included again, and include_once returns TRUE.

The file is 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.

Since PHP 4.0.0 the include_once functionality differs with case-insensitive operating systems, this means that uppercase and lowercase in file names to be included are considered identical.

Under PHP 5.0.0 this behavior was changed, 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.