The packages were moved to http://archives.dotdeb.org/dists/lenny/ but I cant install php 5.3, only php 5.2.
But I realy need the PHP 5.3!
Lenny reached end of life on 2012-02-06, and as such, many sites that previously have offered packages have removed them or moved them to archive sites. http://lists.debian.org/debian-security-announce/2011/msg00238.html
http://archive.debian.org/debian-backports/dists/lenny-backports/ will likely still have a 5.3 package, or if not, you can find the source at http://www.php.net/downloads.php#v5 and the directions on how to compile it at http://www.php.net/manual/en/install.unix.php
This however, should be the least of your worries, as Lenny's end of life on February 6th means you are no longer getting security updates for systems running on Lenny. That's a disaster waiting to happen - Debian already has an extraordinarily long support cycle by Linux standards, well beyond the individual support cycles of many of the packages in debian, and maintaining security in such an environment is no small task. It only takes one mistake or one missed update for disaster to happen, and you've already missed almost 6 months of them.
You need to be working toward upgrading or retiring this machine ASAP. Even behind a firewall, having machines that are seriously out of date on a network is asking for trouble, and if this machine is exposed in any way (even indirectly) to the public internet the problem is that much worse.