RTFM (цитаты)

install.txt из дистрибутива PHP:

IIS 4.0+ (isapi):
1. Copy the php.ini-dist to your systemroot (the directory where you installed windows), rename it to php.ini, and edit it to fit your needs
2. Start the Microsoft Management Console or the Internet Services Manager, located in your Control Panel
3. Click on your webserver, and select properties
4. If you don't want to perform HTTP Authentication using PHP, you can (and should) skip this step. Under ISAPI Filters, add a new ISAPI filter. Use PHP as the filter name, and supply a path to the php 4isapi.dll
5. Under Home Directory, click on the Configuration button. Add a new entry to the Application Mappings. Use the path the php4isapi.dll as the Executable, supply .php as the extension, leave Method exclusions, blank, and check the Script engine
checkbox
6. Stop IIS completely
7. Start IIS again

IIS 4.0+ (CGI)
1. Copy the php.ini-dist to your systemroot (the directory where you installed windows), rename it to php.ini, and edit it to fit your needs
2. Start the Microsoft Management Console (may appear as 'Internet Services Manager', either in your Windows NT 4.0
Option Pack branch or the Control Panel->Administrative Tools under Windows 200).
3. Right click on your Web server node (will most probably appear as 'Default Web Server'), and select 'Properties'.
4. Under 'Home Directory', click on the 'Configuration' button. Add a new
5. entry to the Application Mappings; Use the path to php.exe as the Executable supply .php as the extension, leave 'Method exclusions', blank, and check the Script engine checkbox.
6. Put a .php file under your Web server's document root and check if it works!

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