Picture this situation.
An old (legacy) application has landed on your project pile. It is largely built in php, and you intend on re-writing it in asp.net MVC.
You will therefore need to somehow inform any parties that may be trying to access the old urls ending in .php, that the resource they are looking for has moved permanently. You may also wish to do this for SEO reasons.
This is something that cannot be achieved easily through routing; by default, IIS will not pass requests for resources ending in .php to your application. Your routing will therefore never be put to use for resources ending with .php.
The nicest solution I found to this issue is to setup a list of redirects within the system.Webserver section of your web.config file. The following listing below will send a HTTP Response status of 301 (Moved Permanently) for any requests for index.php and for prices.php:
<system.webServer> <httpRedirect enabled ="true" httpResponseStatus="Permanent" exactDestination="true"> <add wildcard="/prices.php" destination="/prices"/> <add wildcard="/index.php" destination="/"/> </httpRedirect> </system.webServer>
Index.php will now redirect to /, and prices.php will now redirect to /prices.
This code is currently running in the wild on Azure.
If you are unsure if you need a Permanent redirect or not, have a read of this article from Google.