What is the best way to handle sessions for a PHP site on multiple hosts? [closed]

Database, or Database+Memcache. Generally speaking sessions should not be written to very often. Start with a database solution that only writes to the db when the session data has changed. Memcache should be added later as a performance enhancement. A db solution will be very fast because you are only ever looking up primary keys. Make sure the db has row locking, not table locking (myISAM). MemCache only is a bad idea... If it overflows, crashes, or is restarted, the users will be logged out.

Whatever you do, do not store it on the server itself (even if you're only using one server, or in a 1+1 failover scenario). It'll put you on a dead end.

I would say, use Database+Memcache for storage/retrieval, it'll keep you out of Zend's grasp (and believe me things do break down at some point with Zend). Since you will be able to easily partition by UserID or SessionID even going with MySQL will leave things quite scalable.

(Edit: additionally, going with DB+Memcache does not bind you to a comercial party, it does not bind you to PHP either -- something you might be happy for down the road)

Storing the session data in a shared db works, but can be slow. If it's a really big site, memcache is probably a better option.