First off, give serious consideration to the PHP bit - if you've got C# code and presumably therefor skills why throw it away? There are any number of ways to deliver good, modern, flexible, standards compliant websites using C#
So a read of this book might help: Brownfield Application Development in .NET - its not heavyweight and is full of good guidance.
You should be able to deal with many of the issues by systematically refactoring the code, but the database is a bit more of a challenge. You can do various clever things - e.g. if you change table structures you can include views that contain the old structure - but in the end its going to involve pain.
If your really are going to rewrite the app from the ground up then do just that. Ignore what's gone before in terms of code, establish your user stories (you can use the current application to "inform" your stories and the order in which you plough through the backlog), work out your idealised schema and march forward from there...
Can't comment on which approach will get you the desired end result first - but a refactoring approach as per the book will get you visible results (particularly in terms of delivery to your users) a lot sooner.