Some features (such as user authentication) require PHP to be installed as an Apache module.
If you don’t know how to install it, here’s what you need to know.
Excerpted from php's install.txt
First you need to copy php4ts.dll to the windows/system (for Windows 9x/Me)
or winnt/system32 (for Windows NT/2000) directory before overwriting file.
Then add the following two lines to the Apache settings file (httpd.conf)
LoadModule php4_module c:/php/sapi/php4apache.dll
AddType application/x-httpd-php .php
Remember You need to restart the server to take effect, for example
NET STOP APACHE
then
NET START APACHE
and then check phpinfo(). If the Server API is displayed as Apache, it is already running in module mode.