php's header function
This file type is declared functions such as

 header ("content-type: text / html; charset=UTF-8"); 

is a declared file type is text / html format
and UTF8 encoded as a declaration

With this declaration can tell the browser what should be made here actions such as output picture
  you can use image / gif of this type to compare the output
common usage also includes the conversion site such as

 header ("Location:"); 

This way convert the commonly used 301 or SEO Redirect

 header ('HTTP / 1.1 301 Moved Permanently');
header ('Location:'); 

These are commonly used to provide reference usage