What is the difference between PHP require and include?

Question

I know the basic usage of PHP require, require once, include and include once. But I am confused about when I should use them.

Example: I have 3 files, eg: settings.

Answer 1

You don't load settings.php two times, as per PHP documentation on require_once;

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.