The PHP/Java bridge is functional, but we found that it tended to leave around zombie JVM processes as Apache children that have to be kill -9
'd to get rid of. We ended up running a cron job daily to take care of the problem. We only used it for one specific class (an interface class to someone's horrid SOAP endpoint), and it was a tad bit finicky when it came to type juggling, but it did work for us. (The zombie process problem may have been due to the prehistoric PHP version we were using at the time, so it may have been fixed by now.)
I don't have any experience with Gearman and Java, but plenty of it with PHP. It's been a pleasure to work with, for the most part. The most annoying issue is that processing async updates from the caller requires some pretty verbose code to handle all of the possible states. For fire-and-forget and fire-and-get-back-immediate-results RPC, though, it's really hard to beat.