The definitive guide of Symfony 1.2

Chapter 17. Extending Symfony

Eventually, you will need to alter symfony's behavior. Whether you need to modify the way a certain class behaves or add your own custom features, the moment will inevitably happen — all clients have specific requirements that no framework can forecast. As a matter of fact, this situation is so common that symfony provides a mechanism to extend existing classes at runtime, beyond simple class inheritance. You can even replace the core symfony classes on your own, using the factories settings. Once you have built an extension, you can easily package it as a plug-in, so that it can be reused in other applications — or by other symfony users.