Usually this file is stored in the include_path which is set in the php.ini, however, it can be in any other path as long as known.

require_once is similar to require statement, with the only difference being that if the code from a file has already been included, it will not be included again.

The file is included just once.

require_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.