I would like to be able to port some new site pages over to PHP using the same db as in the coldfusion site. Is it possible to have PHP run pages in say other directory and
Yes,
It's technically no different to Windows servers which have both PHP and .Net installed, or a Linux server with Perl and PHP.
Speaking generally, after installing the languages you're using you set up your web server to handle files of different extensions. This is done differently depending on whether you're using IIS or Apache, but you effectively say .php files should be handled by the PHP interpreter and .cfm files handled by the Coldfusion interpreter.