I hear ya. My main work is with C# and Java too, so I stutter a bit when I get into PHP. I also did ASP in the 90's so we're on the same page.
You'll notice that while I helped to create and administer this plugin section, I don't post code, for lack of expertise with PHP and lack of knowledge of this specific application. HOWEVER, my goal is to help facilitate plugin development, and that's what you're trying to do...
To the challenge : One must first understand that the SOAP web service implementation for MantisBT is the most complete, but as we know SOAP is a somewhat deprecated protocol. REST is not as complete, but it is the way to the future. So there are features that are in SOAP, not REST. My quick summary of what we want to do here, and please correct me if this is wrong, is that we want to find a function that's available in SOAP, check to make sure it's not in REST yet, and then port it to REST.
To do that, we need to get more familiar with how other functions are implemented in REST. Find a function that's implemented in both protocols and see how they're doing it. Find another function and see the commonalities. We need to get that pattern into our heads, then apply that to a new REST function.
We need to ensure that we're following the routing/pathing that has been envisioned by other developers contributing to the REST implementation. That means getting into Gitter and opening a discussion with vboctor and dregad.
Federico, are you @FSD-Christian-ISS in Gitter?
Was that you asking recently about POST via REST?
Did you get any helpful info from the REST API docs
Is all of this going down the right path?