Usually in a professional environment when looking for somebody with some knowledge on webservices, what they need is somebody that have actual experience in the company custom web services, more than experience in public web service APIs available the internet. this means, that they need somebody that could develop an application that uses any kind of webservice, with more or less any kind of specification or protocol, or maybe writing or modifying an existing one, or at least has been involved in that kind of development, so he can be familiar with whatever that the staff developing the server can come up with. Note that companies can have a sort of processes in which they communicate with third party applications as client, or have to serve data to them. So, probably if you have made some websites that uses for example Facebook or Twitter APIs, that will be good for you as far as you have really been involved and know exactly what you are doing (and not just reading a reference an using it). Any experience is good, but note that almost anybody with some programing skills, have sometime used some public APIs, so that wont be the difference between two applicants, so if the dont mention some especific API they spect you to know, for sure they are expecting some deeper experience, so you can read a complex WS manual and implement a client which can comunicate with that WS, or even develop a custom service.
in any case you should be familiar with sockets, http request, JSON, HTML, SOAP protocol (in this case, also WSDL), parsing inputs and building well formatted outputs, etc (in php this is easy, but it is very different in C++, Java, etc)
As the good part for you, I can say that developing a web service is not so difficult, it is more or less an application that reads from database as you use to do, and instead of printing hmtl, prints xml or json.
