C++ Difference between const positioning

I'm struggling to get my head around the differences between the various places you can put 'const' on a function declaration in c++.

What is the difference between const at the beginning:

const int MyClass::showName(string id){
...
}

And const at the end like:

int MyClass::showName(string id) const{
...
}

Also, what is the result of having const both at the beginning and at the end like this:

const int MyClass::showName(string id) const{
...
}