using dotnet with visual studio 2015 (and net framework 4.6.1 but its the same with others).  

1) In visual studio 2015, created a library project (in this case a C#)
2) Double click in the project -> build events -> post build

"C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6 Tools\gacutil.exe"  /i "$(TargetPath)"

When the project its compiled, it adds to the GAC automatically. Please changes the bin folder according your installation.

3) in tools -> External tools, add the next external tool
Title: Get Qualified Assembly Name
Command: Powershell.exe
Arguments: -command "[System.Reflection.AssemblyName]::GetAssemblyName(\"$(TargetPath)\").FullName"
User Output Windows : checks

4) run tools -> Get qualified Assembly Name and checks the output windows

5) Creates the next class (Class1 may be its already creates)

namespace ClassLibrary2 // <-- changes it.
{
    public class Class1
    {
        public string Ping(string t1,string t2)
        {
            return "pong "+t1+t2;
        }
    }
}

6) in php

$full="ClassLibrary2, Version=1.0.0.0, Culture=neutral, PublicKeyToken=746927f9726d7222"; // its from the output windows
$class = 'ClassLibrary2.Class1'; // its the namespace and the class.

 $stack = new DOTNET($full,$class);
 $r1="hello";
 $r2="world";
 echo $stack->Ping($r1,$r2);

IMPORTANT NOTE: PHP "caches" the dll library, so every time that the dll library is compiled, the php service should be restarted (restart the apache service).
IMPORTANT NOTE 2: May be you should run Visual Studio as an Administrator.