PHP is primarily a web language, and as such, works well with the usual suspects. Like NogDog said, database of most kinds can be made to interface with PHP, and I find MySQL to work particularly well, especially when you can manage the database with something like phpMyAdmin.
PHP works well with images and you can use it to create all the typical web-based image formats (jpg, gif & png) PDF's can be created and, to a lesser extent, manipulated and working with XML is quite simple now, so working with AJAX based content is easy.
Also, if there's a unique requirement you have that PHP won't do out of the box, it can interact with the operating systems shell to trigger scripts written in other languages. I find a Linux operating system better for this, because tools for working with these other languages are more readily available.