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.
config.php <?php return array("test">1); ------------- //first $config = include_once("config.php"); var_dump($config); $config = include_once("config.php"); var_dump($config); ------------------- output will be array( "test"=>1, )