I am new to Web Service and XML
Just got a job from a client that wants me to build a web app that does what is listed below and other stuff. The scripting language is PHP 5.3

Admin create, modify and delete user
Admin view user details
User login and view their details

At the end of the mail sent to me are 3 information which are listed below
The Query is based on Webservices on XML
The backend Database to be MYSQL
The Operating system to be Cent OS 6.1

I work with MySQL and Google search has shown me that Cent OS is a flavour of Linux but the problem is that I do not understand the term 'The Query is based on Webservices on XML'. I have google for XML and I think it is a technology I can work it. But I did not see any connection about issuing a query inside XML. Please can anybody help out with the term "The Query is based on Webservices on XML" so that I can have an intelligent discussion with client when next I meet with him.