Depends on Your Needs
If you're a developer and need something unique, then go with PHP. Otherwise, WordPress will often do just fine. You can also use a mix where many pages are WordPress and app type pages are in PHP.
As others have said, Wordpress is written in PHP. The better question here is whether to use a commercial content management system or build something yourself. I'd suggest Wordpress for most uses because it will save time and work, but if your needs are too unique, it might be worth building something proprietary.