If you are going to from ASP to PHP, then you will need to recreate your files with a php extension. To do that, you will just need to change the Site Settings from ASP to PHP and the files that gets generated should be php files.
There is no procedure from going to ASP to PHP because you can't really convert the files, you will just have to create the files again and transfer over the functionality. If you use SAP and saved your site as a preset, you could just re-run the wizard after changing your site settings in DW.