I need to implement a web API for a project I'm working on in this period. I read there are many standard protocols to do it: XML-RPC, SOAP, REST. Apparently, the XML-RPC one is the easiest one to implement and use from what I saw, but I didn't find anything about using it to implement an API. Instead I found many tutorial about creating a REST API in PHP, for example. Is there any counter-indication for using XML-RPC to implement a public web API?

Also, more generally speaking, I could (sort of) define a custom protocol for my API, to keep things simpler (i.e. accepting only GET request containing the parameters I need): would this be so bad? Is using a standard protocol a must-do?