You do all that in the context of a single PHP instance and linear code, calling curl_multi handles its own threading, you just get back results, you dont have to store it anywhere outside PHP memory space, and you can configure timeouts and all that stuff, or you can regulate it yourself. The database connector is already doing what it is doing and doing it darn well, and you are still in the same execution context just a few lines down; call out to db, call out to multi for soap requests, handle the results, no syncing issues, no ITC issues, fast, linearly salable. Thread communication, sync, messaging, thread-safe storage, that you would be introduced with threads, and is one that is not there now.
"Since nothing is shared, you'll need some place store that information (ie memached or DB). No idea what you are asking about... -- The trouble with programmers is that you can never tell what a programmer is doing until it’s too late. ~Seymour Cray
