Hook to change HTTP response headers

Question

I want to write a simple Wordpress plugin. The plugin will:

1. Read (if set) a cookie from the visitor
2. Send a cookie (if not set) to the visitor
3. Depending on the cookie value I will redirect the user to a new pages

I found some documentation for getting/setting cookies through WP, and I'm guessing I can just issue a redirect from PHP. So I downloaded the WordPress Boilerplate (WPBP) code and so far so good. But I'm stuck at:

1. Which WP callback should I hook into for my code? (init ?) I found the plugin documentation but I'm struggling to understand where this type of code should hook into.
2. Should I put any of my code into the main/global portion of the WPBP file? Or only in the called back function.
3. This plugin will run on a WP multisite installation. Does that mean my plugin code would run on every site? If so, how could I restrict my plugin code to run on only a particular site? (PHP check for a URI?)

Answer 1

Your questions about how you should structure your plugin are somewhat too broad, but here is a specific answer to the title question.

To change headers before they’re sent, use the wp_headers filter.

function tsg_filter_headers( $headers ) 
{
    // For debug. This will break your page but you will see which headers are sent
    // print_r( $headers );

    // It’s a good idea to leave the admin alone
    if ( !is_admin() ) {

        // Add or redefine 'Content-Location' header
        $headers['Content-Location'] = '/my-receipts/42';
    }

    return $headers;     
}
add_filter( 'wp_headers', 'tsg_filter_headers' );

See the WordPress doc here.