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.

The include and require statements are identical, except upon failure:
- require will produce a fatal error (E_COMPILE_ERROR) and stop the script;
- include will only produce a warning (E_WARNING) and the script will continue