HTTP response codes standardize a way of informing the client about the result of its request.

You might have noticed that the example application uses the PHP header(), passing some strange looking strings as arguments. The header() function prints the HTTP headers and ensures that they are formatted appropriately. Headers should be the first thing on the response, so you shouldn't output anything else before you are done with the headers. Sometimes, your HTTP server may be configured to add other headers, in addition to those you specify in your code.