The .NET Framework is a new computing platform that simplifies application development in the highly distributed environment of the Internet. The .NET Framework is designed to provide a consistent object-oriented programming environment, a code-execution environment that minimizes software deployment and versioning conflicts, which guarantees safe execution of code and to build all communication on industry standards to ensure that code based on the .NET Framework can integrate with any other code.
An assembly is the primary building block of a .NET Framework application. It is a collection of functionality that is built, versioned, and deployed as a single implementation unit containing one or more files. Each assembly contains an assembly manifest.
The Component Object Model (COM) allows an object to expose its functionality to other components and to host applications. Interoperability with COM, or COM interop allows you to create a COM wrapper around the .NET components, which makes Windows look at them as COM objects and thus makes it available to all the calling COM applications.
PHP has built in functionality which uses COM objects, using the COM interop feature of .NET we shall create a wrapper around the .NET assembly and use it in PHP (in fact you could use the same steps to use .NET assembly for VB6 or any other COM applications)
This article assumes that you have fair knowledge of .NET framework, PHP and COM also you have Visual Studio .NET, PHP and IIS installed and working properly.