Highlight menu item based on Yoast Primary Category

yoast-primary-category-menu-item.php

/**
 * Highlight menu item based on Yoast Primary Category
 */
function td_yoast_primary_category_nav_class( $classes, $item ) {
    if ( is_single() ) {
        global $post;
        if ( $primary_category = get_post_meta($post->ID,'_yoast_wpseo_primary_category',true) ) {
            $primary_category_term = get_term_by( 'id', $primary_category, 'category' );
            $menu_locations = get_nav_menu_locations();
            if ( has_term($menu_locations['primary_navigation'], 'nav_menu', $item) ) {
                if (in_array('menu-' . $primary_category_term->slug , $classes) ){
                    $classes[] = 'yoast-primary-category';
                }
            }
        }
    }
    return $classes;
}
add_filter( 'nav_menu_css_class', 'td_yoast_primary_category_nav_class', 10, 2 );