We have tons of very mature PHP4 CMS. And yes some of them even work on PHP5, but honestly those for the most started in the days where code design in the PHP scene was not well established yet. Unit testing was only for the paranoid and coding styles were optional.

He goes on to talk about why reimplementing them in PHP5 would be such a hassle (bad coding practices?) but that there's a light at the end of the tunnel - CMSes like Flux CMS and MODx. Oh, and Java fans will be happy to see his mention of the talk about various CMSes implementing JSR-170 as well.