Since php5 is still in cgi mode, debian doesn't insert this package in the repository (it's unsecure if i well understood).
So you won't find it now.
I don't know if it's the real reason, but it seems like a official one
(i think i've read it on the debian-list)
BTW, if there is a solution to install php4 and php5, i'd be glad to hear it! maybe there is one, i just don't try it yet...
Creak