Require at least PHP 5.2.3. There's not much point increasing the minimum to 5.2...

Require at least PHP 5.2.3. There's not much point increasing the minimum to 5.2.0, as I argued at lists.wikimedia.org/pipermail/wikitech-l/2010-November/050138.html . But requiring 5.2.3 is a significant win. Updates r83217.