On the Debuggable blog there's an interesting post where Felix talks a bit about something I've seen requested quite a bit - a method for locating information in an array. His answer is an XPath-style query system to root out your custom information.

One of the requirements [of the original Set::extract method] was that the new method would need to be faster or at least as fast as the old implementation. My first attempts were big failures. Not only did the solutions I came up with contain tons of bugs. No, they were are also a lot slower the old extract function. A few benchmarks later and I discovered the biggest bottleneck in my implementation: Recursiveness.

He notes that no doing things recursively (not just in this situation, but ever) can help with a speed boost. In his example, a small change made all the differences and the XPath implementation in the CakePHP core makes grabbing information from any array simple.

While the implementation does not support full XPath (and probably won't in future), feel free to make suggestions on additional selectors or the idea in general.