Take a look at the difference of how Php works and how asp.net works on a server in terms of threads and processes. This is one of the major difference between asp.net and php.
so asp.net is compiled and runs on a VM, as threads where as php runs on apache instances as lightweight processes.well php also runs on IIS.
So asp.net is supposed to perform better as the number of request to server increases. However we see that many companies that use php gets it to scale very well using caches etc.
In terms of security, developer should implement security policies etc. However in terms of the programs they both runs in jail.
asp.net is strongly typed (some calls it statically types), with context free grammar. where php is context sensitive and weakly typed.
Php is open source with a big community support. asp.net is from glorious microsoft with fine support and components.
In terms of development, they are equivalent, there is nothing you cant do with one of them that you can do with another one. Both of the languages are Turing Complete.
asp.net apps can be developed using vb or c#, where as php apps are built using php :)
in terms of MVC, asp.net and php has mvc frameworks as well as ORM and many other libraries.
Here is yet another difference, using c# with asp.net, you can use multithreading, threadpools etc. php lacks this.
Exception handling is also different, this is again related to type theory of the languages.