Expect everything to change...

http://net.tutsplus.com/tutorials/php/solid-part-1-the-single-responsibility-principle/

Class and module design is highly affected by The Single Responsibility Principle and leads to a low coupled design with less and lighter dependencies. But as any coin, it has two faces. Excessive SRP consideration can easily lead to premature optimization and instead of a better design, it may lead to a scattered one where the clear responsibilities of classes or modules may be hard to understand