(PHP 4, PHP 5, PHP 7, PHP 8)

The include_once expression includes and evaluates the specified file during the execution of the script. This behavior is similar to the include 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 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.