On the ServerGrove blog today there's a new post talking about TTL collections in MongoDB - queries with a time-to-live set on the indexes - and includes how to use them in PHP.

MongoDB (v. 2.2+) provides a mechanism to expire data from collections by setting a TTL -time to live- on indexes. This is a great feature if you have data that needs to persist in the database for a specific period of time. The TTL feature allows you to expire data using the MongoDB daemon instead of having to create cron jobs to delete expired data. [...] The MongoDB daemon (mongod) checks regularly for documents in collections with a TTL and removes the expired documents.

Using it from PHP is relatively simple - all that's needed is a configuration parameter on the "ensureIndex" call setting an "expireAfterSeconds" value. There's also a bit included in the post for the Doctrine users out there, showing the docblock comment to use to set the value.