Jonathan Wage has a new tutorial on his site showing you how to use the Symfony Expression Language to create simple logic statements. He illustrates with a project they (OpenSky) applied it on - a "reward" rules engine.
We recently adopted the Symfony Expression Language in the rules engine at OpenSky. It has brought a new level of flexibility to our system and creating new logic has never been easier. [...] The expression language allows you to perform expressions that get evaluated with raw PHP code and return a single value. It can be any type of value and is not limited to boolean values.
He starts with a simple example, showing how it can return a boolean based on the results of an evaluation of an array of data. He then takes this up to the next level and use it with a Doctrine object, evaluating the results of methods to apply "rewards" to a user's account. He shows how to define the Doctrine objects with the necessary methods, how to write the rule and a lookup class to find rules that apply to the current situation.