The details of the Spy Master system can be dizzying, but the overall effect is a flexible design that can be easily updated and modified. This system can accept modifications to the underlying database, and can be adapted to an entirely different data set with relatively little effort. Although you didn't learn any new PHP syntax in this chapter, you saw an example of coding for re-use and flexibility. You learned how to use include files to simplify coding of complex systems. You learned how to build a library file with utility routines. You learned how to write code that can be adapted to multiple data sets. You learned how to write code that prevents certain kinds of user errors by limiting choices to legal values. You learned how to build programs that help tie together relational data structures. The things you have learned in this chapter form the foundation of all data-enabled Web programming, which in turn form the backbone of e-commerce and content management systems.