Fragile has a point, Wordpress is designed and featured as a blog system, or content management in the shape of blog.
The title here is a bit problematic - best CMS for PHP website...
If you have a CMS up and running, why should you care if it is using PHP or any other language on the server side?
You worry about the content, not setup, otherwise you would ask - what is the easiest to setup CMS based on PHP?
There are some index sites which gathers PHP open source applications in categories and one of those is CMS (simple, advanced),
the best place is to look there - even if 10 people recommend one content generation / management system over others, it is after all a matter of features / plugins suitable for the client daily activities, and the more intuitive / best marketing software that is selected to be used.
I suggest to focus on PHP development in this forum and not some recommendations for ready made software without comparison - Wikipedia probably has comparison table, and PHP software indexes provide live DEMO for those apps so you don't need to stare at screenshots and read features on dry land.
Zend Support Team