mysql - what is this error Cannot modify header information - headers already sent by that i get when i query my rest web-service built with php

I've built a web-service with framework CakePHP. when i query the service it does return the expected content but, it also gives me an error: Warning (2): Cannot modify header information - headers already sent by. What is this and how can i resolve this?

Answer

Could be that you try to set headers using header function after some of the content is already sent.