could you provide some more insights about your question? Is it more about how to implement a Web Service in PHP or how to write to Hadoop from PHP? What kind of Web Service (SOAP or ReST based) would you like to implement?
There exists a class SoapServer in PHP that allows implementing SOAP services...
Anyway, i'm not sure if SOAP is the right choice here. If you want to write data in a database, and i would interprete Hadoop here as a database, i would suggest to have a look at ReST. It should also be much simpler implementing a ReST service with PHP....