The basic answer to your question is that to do what you want to do, you're going to need to throw away every single line of code you already have, and start the whole thing over again from scratch.
VB.Net is a completely different product to PHP, and moving from a browser-based system to a desktop application is an even bigger leap.
No-one here is going to give you an answer that will really help you make the switch you're asking, because it's too much to answer.
A few small thoughts for you though:
You can install PHP/Apache/MySQL on your local desktop machine. If the problem is that you want to use the software without being on the internet, then you can run it locally on your machine. Even while it's still in the browser.
PHP can be used to write desktop applications. There is a library called PHP GTK which allows you to do this. It will involve a lot of changes to your system, but it will still be PHP, so it might be helpful for you.
If it's just the browser itself that's the problem, you could write a small desktop program in Visual Studio that loads the existing HTML system into a webview control.
You asked about writing ASP code. ASP is also a web-based system, like PHP, so it isn't suitable for writing a desktop application. If you want to do that, you'll need to use VB.Net. You've also used the tag "asp-classic" in your question tags. This is a very old version of ASP, which isn't supported any more. If you do write ASP code, it will be ASP.Net. But you'll be better off writing in VB.Net if you want to write a desktop app.