(PHP 4, PHP 5, PHP 7, PHP 8)
Theinclude_once expression includes and evaluates the specified file during the execution of the script. This is a behavior similar to theinclude expression, 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 returnstrue. 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 theinclude documentation for information about how this function works.