require and include are pretty much the same except that if you use require and the file cannot be found, the whole execution of the script stops. If you use include, even if the file cannot be found, script processing continues.
require_once is the same as require, but if the file has already been included earlier, it won't be included again. You can look all this up at php.net
require()
include()
require_once()
Cgraz
