These days, PHP works well on many different OSes, including Windows (The upcoming version 4.3 will fix certain Windows/IIS problems). But in originally, PHP was written on and for Unix servers.
I personally feel the most stable OS/Server combination for PHP is FreeBSD (
with Apache. FreeBSD has consistently rated near the top of the Netcraft battle for "longest uptimes". (
FreeBSD takes a little more work to understand, but once you understand it, you understand
Unix, and you can make things happen so fast you won't believe it. I can install and configure a full FreeBSD server in less than a 5th of the time it takes to set up and configure a Win2K server.
Also, a note of interest: FreeBSD is open source, and free, just as Linux, but the license is different. The BSD license doesn't attempt to force open source ideology, as does the Linux GPL. This doesn't make much difference to most users, but the BSD license is often considered a little more "business-friendly", allowing businesses to integrate them with proprietary products, without worry of legal repercussions.
Interestingly enough, Apache and PHP are also both distributed with BSD-style licenses.
I also recommend PostgreSQL (
as the most advanced open-source database system to use with PHP. It far outstrips MySQL for enterprise-level features, and even competes with some of the large commercial DBs. PostgreSQL runs well on FreeBSD, as well as on any other Unix platform. Of course, PostgreSQL is also distributed with the BSD-style licence, since it was created by the same great university as FreeBSD (Berkeley) ;-). -------------------------------------------
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