Show Yoast primary category on WPSP

gistfile1.txt

add_filter( 'wpsp_terms_output', 'display_primary_cat_yoast', 15, 1 );

function display_primary_cat_yoast($output){
    // SHOW YOAST PRIMARY CATEGORY, OR FIRST CATEGORY
    $category = get_the_category();
    $useCatLink = true;
    // If post has a category assigned.
    if ($category){
        $category_display = '';
        $category_link = '';
        if ( class_exists('WPSEO_Primary_Term') )
        {
            // Show the post's 'Primary' category, if this Yoast feature is available, & one is set
            $wpseo_primary_term = new WPSEO_Primary_Term( 'category', get_the_id() );
            $wpseo_primary_term = $wpseo_primary_term->get_primary_term();
            $term = get_term( $wpseo_primary_term );
            if (is_wp_error($term)) { 
                // Default to first category (not Yoast) if an error is returned
                $category_display = $category[0]->name;
                $category_link = get_category_link( $category[0]->term_id );
            } else { 
                // Yoast Primary category
                $category_display = $term->name;
                $category_link = get_category_link( $term->term_id );
            }
        } 
        else {
            // Default, display the first category in WP's list of assigned categories
            $category_display = $category[0]->name;
            $category_link = get_category_link( $category[0]->term_id );
        }

        // Display category
        if ( !empty($category_display) ){
            if ( $useCatLink == true && !empty($category_link) ){
            $output = '<span class="post-category">';
            $output .= '<a href="'.$category_link.'">'.htmlspecialchars($category_display).'</a>';
            $output .= '</span>';
            return $output;
            } else {
            $output = '<span class="post-category">'.htmlspecialchars($category_display).'</span>';
            return $output;
            }
        }  
    }
}