Well, besides the fact that the language is massively easy to learn?
1. Good and decent object orientiedness. I've been able to prepare a lot of middleware classes in a recent project.
2. A function for almost everything. Because this is a web language, almost all the web-related tools are built in. Simple things like opening some web page across the world as a file, saved me a lot of trouble once or twice. I remember having a CVS file -- PHP has a function to read CVS files and put each record in an array! Normally you'd have to code that in VB/Java.
3. Easy to learn language. I was in charge of a bunch of developers recently. They all were able to pick up the language in days.
4. Built in database integration with all the major commercial and open-source database servers.
5. Cross platform -- so not much code changes necessary to port.
6. Although you can embed PHP in HTML, lots of libs exist to allow PHP to be a good middle tier.
7. PHP 4 - Nuff said.
PHP, Perl, SQL Programming at http://www.mentalobjects.com