include vs require and require_once vs require

require vs. include

The require() function is identical to include(), except that it handles errors differently. If an error occurs, the include() function generates a warning, but the script will continue execution. The require() generates a fatal error, and the script will stop.

require_once vs. require

The require_once() statement is identical to require() except PHP will check if the file has already been included, and if so, not include (require) it again.