Is there a simple PHP development server?

Use XAMPP: http://www.apachefriends.org/en/xampp.html Its easy to install and config.

EDIT:
since PHP 5.4 (which was released 2 years after this answer) there's a built-in web-server. Today this is inaccurate, please use the method provided by k0pernikus in his answer.

Try using batch file

1. Open notepad
2. php -S localhost:8000
3. save file as .bat extension => server.bat
4. now click on server.bat, your server is ready on http://localhost:8000.

DEP

if you got error php not recognize any internal or external command then goto environment variable and edit path to php.exe

"C:\wamp\bin\php\php5.4.3"

Actually PHP 5.4.0 recently presented its built-in web server:

cd ~/public_html
php -S localhost:8000

If it's a feasible option on whatever platform you're using, PHP 5.4 has a Django-like development server.

You may also want the snippet I use to detect the development server and replicate the two most common uses of mod_rewrite. (Hiding index.php in URLs and passing static files through to be served)

(Yes, there is a built-in command-line switch for that, but it doesn't quite match the mod_rewrite snippet most people use and caused spurious 404 errors for me because of that mismatch.)