The require_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the require() statement, with the only difference being that if the code from a file has already been included, it will not be included again.
require, require_once will throw an error if the file required is not found but include just throw a warning.
require - File must be included or the script will exit with a fatal error require_once - Same as above, except it can only be included once include - same as require, except script will continue if the file is not found, and only produce a warning include_once - same as require_once except script will continue if the file is not found and only produce a warning Click on the links above for more information about each. That is what the manual is there for