PHP - 使用经过身份验证的请求创建API

Question

I've created a simple PHP restful API like:

Register

api?action=register&username=xcodeWarrier&email=[e-mail]&password=abc

To post content

api?action=postContent&content=123&username=xcodeWarrier

How do I protect the postContent API call to prevent anyone posting content as xcodeWarrier.