To be honest, I do not know how to use PHP for this purpose, but if I understand you well, you would like to make calls from a website. Right? If so, I think you will need a webphone (it is actually a softphone embedded to a website).
I have some knowledge of VoIP development in C#. To implement a webphone in C#, you can use the Silverlight or the Flash technology. After some googling I found this website that says:
For using web telephony, you and your partner will need webphones to
talk through. The webphone is a client-server application. The client
part is a program integrated into a website. This client is connected
to the webphone server that is usually a console application. The
server forwards the call to another party, which can be another
webphone, a softphone, a regular IP phone or even a call center. Using
webphones is a very convenient way of communicating with your
customers, as they can call you directly from your webpage.
The mentioned page provides more details about Silverlight and Flash and it also offers some C# code examples.
I hope it helped you to get started.