How to add code to HTTP header

WordPress has the action send_headers, which you can use to make it send additional headers: 

add_action( 'send_headers', 'add_header_acao' );
function add_header_acao() {
    header( 'Access-Control-Allow-Origin: *' );
}

See the documentation http://codex.wordpress.org/Plugin_API/Action_Reference/send_headers

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