$original_value = 'foo';
$filtered_value = apply_filters( 'wpd_filter_function', $original_value );

function wpse203948_header_cart(){
    echo 'something';
}
add_action( 'wpse203948_header_cart', 'wpse203948_header_cart' );

// applying it
do_action( 'wpse203948_header_cart' );