I'm not sure if this is the right place to post a question regarding web services...forgive me if not.
I've been in web development for a while now and have heard a lot of discussion about web services. I understand that a web service is pretty much a web-based API--the client submits a request, and the server submits a response. Great. I've been doing that for years now. If I have a database with data, I create a PHP script that will serve as an interface to that data. I submit a request to the PHP script, and it returns my data in an XML format that I can then parse, display, or whatever.
What I would like to know is what makes a web service different from that PHP script that serves as an interface to my database. Is it a simple web service? If not, why not? How do WSDL and SOAP fit in to all this?