I guess I am being too obscure, in part it is because if I was in a Windows environment, the superficial simplified answer would be "use .Net".
Let's give an example. I have a prototype bookings system for meetings, written in PHP4 using PostGres, been running happily for ages, but it is totally web page based. Works nicely for the 40 or so users who use it, been sitting on Redhat 7.2 for 3 years, though recently moved onto Mandriva 2005 SE. It is poorly structured, with no separation between GUI and backend updating.
Let's stay I want to rework this to cater for a large organisation, who are likely to want to not only use screens, but pass in bookings from other systems, interface to other systems and so on. (but mainly I want to prototype something to understand the technology
I need a chunk of backend server code which should be able to service both web services and web pages. In the Windows envronment, I might have used COM or one of its variants - now I would use .Net which is fairly painless for doing that sort of thing. On Linux I would want to run some sort of service, but I don't really want to work down at that level - what is around to protect me from working at that low level without rewriting a chunk of low level services?