In the properties of the new site, on the ISAPI filters tab, do you see the "PHP" ISAPI filter you installed? If you click on it, what does it say the status is (down below)?
On the Home Directory tab, what are the settings for Execute permissions and Application pool?
In that same Home Directory tab, there is a configuration button to the right of the Execute permissions dropdown. Click on it. Have you mapped the .php extension in here? If not, click the Add button, type ".php" for the extension, the full path to the ISAPI .dll file for Executable, leave Verbs to All verbs, and make sure that both checkboxes at the bottom are ticked.