1. Ample project, http://www.ample-project.net/
2. Optimost, http://www.optimost.com/
3. phpAspect: Aspect oriented programming for PHP, http://phpaspect.org/
4. Vertster, http://www.vertster.com/
5. Software product lines: practices and patterns. Addison-Wesley Longman Publishing Co., Boston (2001)