For more examples on using require_once() and include_once(), look at the PEAR code included in the latest PHP source code distributions.
As mentioned in the require_once() description, the include_once() should be used in the cases in which the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.