include_once

(PHP 4, PHP 5, PHP 7)

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, and include_once returns TRUE. As the name suggests, the file 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.

Note: 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 <?php 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.

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https://secure.php.net/manual/en/function.include-once.php