What is the difference between include(), include_once() and require_once() ?

The include() statement includes and evaluates a specified line i.e. it will include a file based in the given path. require() does the same thing expect upon failure it will generate a fatal error and halt the script whereas include() will just gives a warning and allow script to continue. require_once() will check if the file already has been included and if so it will not include the file again.