include()
The include() function takes all the content in a specified file and includes it in the current file. If an error occurs, the include() function generates a warning, but the script will continue execution.

include_once()
File will not be included more than once. If we want to include a file once only and further calling of the file will be ignored then we have to use the PHP function include_once().

require()
The require() function is identical to include(), except that it handles errors differently. The require() generates a fatal error, and the script will stop.

require_once()
The required file is called only once when a page is open and further calling of the file will be ignored.