(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.