Actually, I would call 5.3 as a fairly major jump - namespaces, closures (Lambda/anonymous functions), late static binding, const's allowed outside of classes.

Actually, I would call 5.3 as a fairly major jump - namespaces, closures (Lambda/anonymous functions), late static binding, const's allowed outside of classes. There are several popular PHP frameworks that require v. 5.3+. Some people even go so far as to call the 5.3.x branch “php 6 without Unicode”.