It happens many times, like now, where I'll be sitting at more than one computer, and will want to share work between them, especially without swapping keys or mouse. I already use Synergy, perhaps more correctly, quicksynergy, to share the mouse and keyboard, but I've been looking for a way to "move" an app from one desktop to the other. I think I found it.
Searching off-and-on, I eventually stumbled upon xpra, a handy little X environment that hosts the application and allows other desktops (including the localhost) to attach and interact with the applications running on it. This allows a desktop that is not running the application to still display the application's interface, as long as the application plays nice.
The documentation at the xpra website is pretty sparse. While looking for it, I found another link to a package at http://xpra.devloop.org.uk/ that does a nice apt-friendly installation for you. Once installed on all of the machines in question, it's about as easy as this.
On the host (the machine on which the application is going to run), start the xpra with a command like xpra start :15 and the environment will fire-up. From there, it's a matter of telling your X apps to start on that desktop. Depending on the app, it could be as easy as DISPLAY=:13 && appname or even specifying the display on the command line, if the app allows it. This will start the app on the xpra "desktop."
To then use the app, you attach to it using xpra attach :15 or when connecting from another system, somethng such as xpra attach ssh:hostname:15 and all of the windows on the xpra environment will spring open.
I've not found the nifty way to close the shell window which is doing the attach. Control+C works, but offers a bunch of warnings as the PHP app dies its apparently horrible death. Also, it seems that when another client attaches, any currently attached client simply loses its connection. This can be a nice way to tell if someone's swiping your app, but it's a little unfriendly because your app just got swiped.
I'm going to play with it a little more, and hopefully have some of the annoyances figured out. I'm sure it's as much me not knowing what's going on as it is xpra not doing what I want it to do.