// Modified from http://www.rarescosma.com/2010/11/add-a-class-to-wp_nav_menu-items-with-urls-included-in-the-current-url/
// Need to add a class (using 'current-menu-item' to the nav menu to highlight it
// when on custom post type pages (e.g. "/movies/" )
// Calendar needs to look for "Calendar, Venues,
// (and in this case Organizers, if you've updated the post_type with a slug)
function add_parent_url_menu_class( $classes = array(), $item = false ) {
    // Get current URL
    $current_url = current_url();

    // Get homepage URL
    $homepage_url = trailingslashit( get_bloginfo( 'url' ) );

    // Exclude 404 and homepage
    if( is_404() or $item->url == $homepage_url ) return $classes;

    // if $item->url can be found in $current_url
    // OR if $current_url (haystack) contains organizer, venue, events
    // AND $item->url is blah/calendar, add class
    if ( strstr( $current_url, $item->url) || ( ( strstr( $current_url, '/events/' ) || strstr ($current_url, '/organizer/') || strstr($current_url,'/venue/') )  && strstr( $item->url, '/calendar/') ) ) {
        // Add the 'current-menu-item' class
        $classes[] = 'current-menu-item';
    }

    return $classes;
}
add_filter( 'nav_menu_css_class', 'add_parent_url_menu_class', 10, 2 );

// this is a helper function to the above add_parent_url_menu_class;
// it checks the current url and pulls all the universal stuff out
function current_url() {
    // Protocol
    $url = ( 'on' == $_SERVER['HTTPS'] ) ? 'https://' : 'http://';

    $url .= $_SERVER['SERVER_NAME'];

    // Port
    $url .= ( '80' == $_SERVER['SERVER_PORT'] ) ? '' : ':' . $_SERVER['SERVER_PORT'];

    $url .= $_SERVER['REQUEST_URI'];

    return trailingslashit( $url );
}