Have you tried require()? It works similar to include().
(From PHP.net)
require() and include() are identical in every way except how they handle failure. include() produces a Warning while require() results in a Fatal Error. In other words, don't hesitate to 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. Be sure to have an appropriate include_path setting as well.
Plus, if you have class definitions that are commonly used in other classes, why not just make a 'super class' parent with these definitions, and derive your other classes from the 'super class'?