Vic Cherubini has written up a tutorial on his site showing you how to write functional tests for Symfony services in your application. He provides a practical example of testing a basic Symfony service and the configuration/code to go with it.

The dependency injector is an amazingly simple and flexible addition to Symfony, and one you should be using to properly structure your application. But what happens when you want to write a functional (or integration) test for a service that depends on another service? This article will show you an easy way to test complex services.

He sets up a simple InvoiceGenerator service that takes in a Doctrine entity manager and a "payment processor" instance. He stubs out a simple PaymentProcessor class and shows the configuration needed to set it all up for correct injection. He then gets into the testing of this setup, creating a simple test case that requests the invoice generator from the service container. In this call the services_test definition overrides the default and injects the test payment processor instead of the actual one.