From the phpinfos stats made in november 2006, we could extracts figures of MySQL versions being used with PHP.
The evolution of mysql libraries has two steps : the server must upgrade, and the libraries too. This way, they will take advantage of the new identification mecanism, which is much safer than the old one.

But sometimes, the SQL server or the application do not evolve, and the whole architecture is compelled to use the old identification mecanism, which is less safe.