When you changed your system from .NET to PHP you had several choices:
- use the .aspx extension for your PHP files and set the server configuration to parse .aspx files for PHP content. Continue to link to exactly the same .aspx URLs as before. - use .php files on your server, but continue to link to .aspx URLs. Set a rewrite so that when a .aspx URL is requested, content is served from the matching .php file. - use .php files on your server, and change all the links on your site to point to these new .php URLs. Set up a redirect so that if any .aspx URL is requested, browser is redirected to the new .php URL.
The last one of these will result in the site losing rankings because by giving your pages new URLs, you are in effect starting again with a new site.
The first two options would see no change in rankings or traffic because the URLs that users "see" and "use" remain exactly the same, all of the "magic" being done out of sight inside the server.