PHP initially started on unix platform, but it is very portable and runs on MS Windows and MS IIS webserver. Today PHP has a large user base on MS Windows 2000/NT/95/98, You will find a huge collection of tools for PHP under MS Windows platform.
Many PHP programmers develop code on MS Windows and deploy on large linux servers like IBM mainframe running linux, Compaq DEC Alpha and Sun sparc.
A great advantage is that since PHP also runs on Unix/Linux, developers on unix platform "cash on" the user base of PHP under MS windows as the PHP code developed under MS Windows can be used on unix/linux without any code change!!
PHP itself is written in 100% "C" langauge, and hence it runs on a very wide variety of platforms like BeOS, Unix, MS Windows, Apple Macintosh, IBM OS/2 and on many more operating systems.
PHP is very fast and is much faster than Java. For web development, forget Java/JSP, it is PHP, PHP and PHP everywhere!! PHP is also becoming a general purpose object oriented scripting language