How to add .Net dll (C# - Custom class library dll) reference in php (codeigniter)

Where I Have to add C# dll library [for PHP]? I tried in php/ext folder (is it right)?

Dll contains method name: TestMethod/HelloMethod

Output: Hello world.

$class1 = new DOTNET("SampleTest,Version=1.0.0.0, Culture=neutral,PublicKeyToken=''","SampleTest.TestMethod");
echo $class1->HelloMethod();

Error Message : Failed to instantiate .Net object [CreateInstance] [0x80070002] The system cannot find the file specified.