I'm not sure whether you are still interested in feedback on php in the testing repo but here goes anyway. I have used the testing repo to upgrade php for use with RoundCube which requires php5.2 or greater and it runs without problem.
However during configuration, the programme complained that it couldn't find php-mcrypt (not surprising as it's not in the testing repo). On checking the logs, I see :
PHP Warning: PHP Startup: mcrypt: Unable to initialize module Module compiled with module API=20050922, debug=0,thread-safety=0 PHP compiled with module API=20060613, debug=0,thread-safety=0 These options need to match
No doubt this is because I had the mcrypt module for php5.1.6 installed. In which case, can I make a request for an updated module? Not high on your priorities I guess.
KeithW