Grab The PHP writing plugin for Eclipse from the update site and check it out if it works for you!
Sorry, but java6 and eclipse 3.3 only! (jsr-223 scripting engine starts from java 6)
Have fun and let him know if you find some cool usage for it. Commenting at his blog is a good start.
I had to disable comments at my blog :(
A cool feature would be to write a small snippet that sends selected text to some rafb/etc pastebin and prints the url to the eclipse console view.