I'm attaching the shell script I used to do this...Slightly modified from other variations online. Obviously don't run this without understanding it- it will remove your existing php packages

Then if you need to install any additional php packages, just install them with ( notice the -t karmic option )

For more info:
http://jtrancas.wordpress.com/2010/05/04/cakephp-1-1-and-php-5-3/
http://ubuntuforums.org/showthread.php?t=1459163&page=2

UPDATE: I gave up on this approach and moved back to Eclipse Europa 3.3

Much to my dismay, there were just too many bugs trying to get FB to support 3.5 correctly. I had it mostly working, but there are unresolved issues with the ProblemManager Class that caused consistent "internal build error" messages and generally completely unacceptable error reporting.

I don't know about you but the scope of my Flex work is such that I need top-notch error-reporting. And I really don't like having to apply multiple patches to get my IDE to a half-functional state.