On the SitePoint PHP blog today, there's Harry's opinion on telling "when PHP5 is safe to use".

The answer is very simple - watch John.

When John migrates, to me it means two things in particular;

• If there is a performance hit vs. the older PHP version, it will be an acceptable one.
• There are no show-stopping bugs.

It's an interesting standard to set for the adoption of something as beneficial as the move from PHP4 to PHP5, but it is one way to judge the effectiveness. When you see the major applications tailoring their setups for PHP5 environments, it might be time to consider jumping ship yourself...