I guess you don't need Content-Type header only, but entire POST data with fields which are URL encoded etc.
You need to either use RAW PHP or curl (preferred).
So, that's if your app plays role of a client (i.e. you are sending data to an external API). On the contrary, if your app is both a client and a server (regular web app), all you need to do is to build HTML form with POST method, which will set Content-Type: application/x-www-form-urlencoded.
These are very foundations of the HTTP protocol.
@zhegwoodheader() in PHP is not a method but function. It's the same as using setHeader() method in Phalcon's Response component, which is always more favoured solution (i.e. if you use a framework, use it on each stack level).