PHP help
Installing PHP
Install the Apache web server on your computer--it will run on XP, and it comes with PHP. For configuration, you edit one or more configuration files with a text editor. Check the Apache website for installation/getting-started documentation.
Install the IIS 5 web server that comes with XP. Use Control Panel>Add or Remove Programs and select the Add/Remove Windows Components. Test to see that you have a functioning website at http://localhost. The site files will be in C:\Inetpub\wwwroot. By default, this will support ASP for server-side scripting. Then find, download and install PHP for IIS 5. (Not IIS 7 or later...that only runs on Windows Server, not XP.)
Another option is to use VirtualBox to run a Linux server on a virtual machine. You can still use your XP web deveopment tools and test over a virtualized TCP/IP link. Ubuntu is probably the most popular Linux distro right now. Scientific Linux, which is basically Red Hat Enterprise--minus RHE proprietary code--recompiled with only the free, open-source components os also popular.
Either with a virtual machine or a local Apache server, it's probably best to use Apache. That's what most (all?) of the websites serving PHP on the Internet will be using.