What are the differences between require and include, include_once and require_once?

The include() statement includes
and evaluates the specified file.The documentation below also applies to
require(). The two constructs
are identical in every way except how they handle
failure. include() produces a
Warning while require() results
in a Fatal Error. In other words, use
require() if you want a missing
file to halt processing of the page. 
include() does not behave this way, the script will
continue regardless.

The include_once()