Is %HOMEDRIVE% valid in web.config?

trying to add a handler to my web.config file, but using a windows variable doesn't work. How can I use %HOMEDRIVE% or something similar in the scriptProcessor attribute?

        <handlers>
            <remove name="PHP_via_FastCGI" />
            <add name="PHP_via_FastCGI" path="*.php" verb="GET, PUT, POST, DELETE, HEAD, OPTIONS" modules="FastCgiModule" scriptProcessor="%HOMEDRIVE%\php\php-cgi.exe" resourceType="Either" requireAccess="Script" />
        </handlers>