I don't particularly like the idea of carrying two PHP modules in Fedora Core at least, one is big enough as it is. (you can't load two PHP modules into Apache at the same time so there's no way of making both work "out of the box" anyway)
I plan to prepare packages of 5.0.0 which people can test out; maybe we can upgrade to PHP5 during the FC4 release cycle. Maybe later.
This route works, too, but I wouldn't jump on the php5 bandwagon quite yet for FC3. If it can be provided in extras with a "conflicts" flag set so they aren't both installed on the same machine, I think that would be good enough for most people.
Regards,
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Konstantin ("Icon") Ryabitsev
Duke Physics Systems Admin, RHCE