I have the possibility of migrating a site from php to asp.net. Currently the site ranks very high in Google for various different searches. I would like some guidance on how to approach the redevelopment of the site to asp.net. It wouldn't be a straight migration it would be a total redevelopment, whilst still ensuring that the website retains it high ranking within search engines.

Many thanks for your help.

Once you have migrated your content over to the new site, then create a PHP algorithm that will figure out the new link that you intend to use, and redirect using a 301 header (Moved Permanently).  It's possible to even change the PHP extension to be handled as an ASP.NET page by your server, so you could code the algorithm in .NET if you choose to do that once you have finished the redevelopment of course.

This will tell search engines that the content has moved to the ASP.NET pages, which should help in your page rankings as long as people still link to either page, and it will also keep people from getting 404 Page Not Found errors when you have migrated the system.

You'd have to put something like this in your web.config

<urlMappings enabled="true">
    <add url="~/index.php" mappedUrl="~/Default.aspx" />
    <add url="~/articles.php" mappedUrl="~/articles/Default.aspx" />
</urlMappings>