One of my first posts here was about how surprisingly inefficient PHP arrays can get. Today I learned of a solution that is probably a lot better than my PHP string serialisation. It's an extension called intarray.
The extension exposes integer-only arrays as strings to PHP but provides several useful methods for interacting with them such as sort, slice and binary search. This means if you are using PHP arrays to store sets of integers, you will likely see a very large improvement in speed and memory usage using this extension.
I've yet to do any real benchmarking but I thought I'd post this as a follow-up from my original post. I know at least one very large site who has used this extension in production with no issue although I obviously urge anyone to evaluate stability etc of any software themselves before deploying.