The 3 most widely used PHP CMS's presently are Wordpress, Joomla! and Drupal - quite in that order as well.
Wordpress - Easy to install, good basic features, relatively safe ( without the plugins, though we all need them to power up wordpress) and easy to handle as well. Good for those wanting a basic CMS and putting content up on there sites. Very good SEO with plugins.Google's pet.
Joomla! - Relatively complex compared to Wordpress but it is a full fledged CMS with advanced features like Social Networking, Video sites and with a huge community following as well. However, not the best for SEO , even with plugins like sh404sef it has inherent drawbacks of duplicate content and is a pain as far as solving these issues is concerned. External plugins make it unsafe, core is very safe and upto date.
Drupal - Not for the faint hearted. The learning curve is pretty high but once you know it in an out , you can only love the power it gives with the flexibility it comes with. Used by power portals and firms , it can be easily leveraged for SEO ( very good at that) and is relatively much safer then the other two. Its only con is not easy to work on and the UI lacks.
Having worked on all 3, for me it depends on the need. If its a powerful system, then drupal . If i wanna keep it simple, then Wordpress and for everything in between

, Joomla! .