A quick way to add a URL redirect feature to Wordpress categories

wordpress_category_redirect.php

<?php namespace content;
/**
 * Category_Add_Field
 * 
 * redirect() method is called in the header.php file
 * if the user specifies an external URL in the category field 
 *
 * @package class for creating dynamic Wordpress metaboxes
 * @author Andy Walpole
 * @copyright A Walpole
 * @version 2013
 * @access public
 * @license the same as Wordpress
 */

class Category_Add_Field {

    /**
     * Category_Add_Field::__construct()
     */
    public function __construct() {

        add_action('category_add_form_fields', array(&$this, 'taxonomy_add_new_meta_field'), 10, 2);
        add_action('category_edit_form_fields', array(&$this, 'taxonomy_edit_meta_field'), 10, 2);
        add_action('edited_category', array(&$this, 'save_taxonomy_custom_meta'), 10, 2);
        add_action('create_category', array(&$this, 'save_taxonomy_custom_meta'), 10, 2);

    }

    /**
     * Category_Add_Field::taxonomy_add_new_meta_field()
     * 
     * @return string
     */
    public function taxonomy_add_new_meta_field() {

        $field = '<div class="form-field">';
        $field .= '<label for="term_meta[custom_term_meta_url]">';
        $field .= 'Enter a full URL here';
        $field .= '</label>';
        $field .= '<input type="text" name="term_meta[custom_term_meta_url]" id="term_meta[custom_term_meta_url]" value="">';
        $field .= '<p class="description">';
        $field .= 'Leave blank if no redirect required';
        $field .= '</p>';
        $field .= '</div>';

        echo $field;

    }


    /**
     * Category_Add_Field::taxonomy_edit_meta_field()
     * 
     * @param string $term
     * @return string
     */
    public function taxonomy_edit_meta_field($term) {

        // put the term ID into a variable
        $t_id = $term->term_id;
        // retrieve the existing value(s) for this meta field. This returns an array
        $term_meta = get_option("taxonomy_$t_id");

        $field = '<tr class="form-field">';
        $field .= '<th scope="row" valign="top"><label for="term_meta[custom_term_meta_url]">';
        $field .= 'Enter a full URL here';
        $field .= '</label></th>';
        $field .= '<td>';
        $field .= '<input type="text" name="term_meta[custom_term_meta_url]" id="term_meta[custom_term_meta_url]" value="';
        $field .= esc_url($term_meta['custom_term_meta_url']) ? esc_url($term_meta['custom_term_meta_url']) : '';
        $field .= '">';
        $field .= '<br><span class="description">';
        $field .= 'Leave blank if no redirect required';
        $field .= '</span></td></tr>';
        echo $field;

    }


    /**
     * Category_Add_Field::save_taxonomy_custom_meta()
     * 
     * @param string $term_id
     * @return 
     */
    public function save_taxonomy_custom_meta($term_id) {

        if (isset($_POST['term_meta'])) {

            $t_id = $term_id;

            $term_meta = get_option("taxonomy_$t_id");

            $cat_keys = array_keys($_POST['term_meta']);

            foreach ($cat_keys as $key) {

                if (isset($_POST['term_meta'][$key])) {

                    $term_meta[$key] = $_POST['term_meta'][$key];
                }

            }

            // Save the option array.
            update_option("taxonomy_$t_id", $term_meta);
            
        }

    }

    /**
     * Category_Add_Field::redirect()
     * 
     * @param string $id
     * @return redirect
     */
    public function redirect($id) {

        $term_meta = get_option("taxonomy_" . $id);
        
        if ($term_meta != false && (isset($term_meta['custom_term_meta_url']) && $term_meta['custom_term_meta_url'] != '')) {

            ob_start();
            wp_redirect($term_meta['custom_term_meta_url'], 302);
            ob_end_flush();
            exit;

        }

    }

} // end class

new \content\Category_Add_Field;


// use this class by calling the redirect method at the top of the header file like so:

if (class_exists('\\content\\Category_Add_Field')) {

    $cat = get_query_var('cat');
    
    // category redirection if external URL is set in the category page

    if ($cat != '') {

        \content\Category_Add_Field::redirect(get_query_var('cat'));

    }

} else {

    echo 'No class called Category_Add_Field has been found';

}