$ php --ri extension
News, news, yawn.
Daniel Stenberg, head of the cURL project, accepted a patch for sub second time out support within libcurl. That means that you can use float values as time outs with a millisecond resolution, as soon as you use libcurl >= v7.16.2, that is, a minimum supported time out of 0.001 seconds (which is not reasonable, just in case you wonder). On a side note, you should build libcurl with c-ares to get working sub second DNS lookup time out support, AFAICT.
Is there an option in pecl_http that I'm overlooking that would be the equivalent of STREAM_CLIENT_PERSISTENT?
I was already about to answer that libcurl already does that all for us, realizing that we destroy the used CURL handles at the end of each PHP request at the latest, thus killing any alive connections. This happened about two weeks ago, and pecl/http 1.5, of which RC1 had just been released, will support per process persistent CURL handles. It can be enabled at compile time only, as it's a quite intrusive feature. Yet I'm still unsure about the implementation of this feature and it might change in the future to become a bit more user-friendly.
To use it, there's nothing else to be done than tossing --enable-http-persistent-handles at configure.