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
http://www.nickveenhof.be/blog/reverting-or-downgrading-php-53-52-ubuntu...