Quoting Rene Pijlman, from the post of Fri, 06 Aug:
Unfortunately, there is no backport of PHP 4.3.x for woody on www.backports.org.
I use this: http://dotdeb.org
I find their response time to erratas is also very quick. I run MW 1/3beta3 on it with no ill effects.
What's the best way to proceed? Install PHP 4.3 from source on woody? Or upgrade the machine to Debian sarge?
Do NOT upgrade a production server to testing! It has no security updates! I had a major break-in when I upgraded to woody when it was still in testing. a silly and easy crack that was fixed in stable a week before this happend.