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get_header is a hook that gets run at the very start of the get_header function call. If you pass in the name for a specific header file into the function get_header(), like get_header( 'new' ), the do_action will pass in the same name as a parameter for the hook. This allows you to limit your add_action calls to specific templates if you wish. Actions added to this hook should be added to your functions.php file.

Note: This hook is best to use to set up and execute code that doesn’t get echoed to the browser until later in the page load. Anything you echo will show up before any of the markups is displayed.