Wordpress - Order get_terms() By Custom Field

SOLUTION - Updated: 03/03/2015 - Thanks to /u/G.M.

The below adds a new field to the Term Edit Page and saves the value into the un-used ( for the moment anyway ) term_group field which can then be used to order terms. In the below hooks ( actions ) you'll need to replace TAXONOMY_SLUG with your actual taxonomy slug.

/**
 * Append Fields To Term Edit Page
 * @param Term Object $term
 * @param string $taxonomy
 */
function term_order_field( $term, $taxonomy ) {
  ?>
    <tr class="form-field">
        <th scope="row" valign="top">
            <label for="meta-order"><?php _e( 'Category Order' ); ?></label>
        </th>
        <td>
            <input type="text" name="_term_order" size="3" style="width:5%;" value="<?php echo ( ! empty( $term->term_group ) ) ? $term->term_group : '0'; ?>" />
            <span class="description"><?php _e( 'Categories are ordered Smallest to Largest' ); ?></span>
        </td> 
    </tr>
  <?php
}
add_action( 'TAXONOMY_SLUG_edit_form_fields', 'term_order_field', 10, 2 );

/**
 * Save Term Order
 * @param int $term_id
 */
function save_term_order( $term_id ) {
    global $wpdb;

    if( isset( $_POST['_term_order'] ) ) {
        $wpdb->update( 
            $wpdb->terms,
            array(
                'term_group' => $_POST['_term_order']
            ),
            array( 
                'term_id'    => $term_id
            )
        );
    }
} // END Function
add_action( 'edited_TAXONOMY_SLUG', 'save_term_order' );

Add Term Order cell to Term List

/**
 * Add Column To Show 'Term Order' Field
 * @param array $columns
 * @return array $columns
 */
function term_order_header( $columns ) {
    $columns['order'] = '<center>Order</center>';
    return $columns;
}
add_filter( 'manage_edit-TAXONOMY_SLUG_columns', 'term_order_header' );

/**
 * Give 'Term Order' Column A Value `term_group`
 * @param string $empty
 * @param string $col_name
 * @param int $term_id
 * @return string
 */
function term_order_header_val( $empty = '', $col_name, $term_id ) {
    if( isset( $_GET['taxonomy'] ) && 'order' == $col_name ) {
        $term = get_term( $term_id, $_GET['taxonomy'] );
        return "<center>{$term->term_group}</center>";
    } else {
        return '0';
    }
}
add_filter( 'manage_TAXONOMY_SLUG_custom_column', 'term_order_header_val', 10, 3 );

Hook Documentation

• action {$taxonomy}_edit_form_fields
• action edited_{$taxonomy}
• filter manage_{$screen->id}_columns
• filter manage_{$this->screen->taxonomy}_custom_column

---

Below is an out-dated solution that works better if you have multiple Term Meta Values you would like to save. If you only need to save Term Order, the above solution is best.

/** Add New Field To Category **/
function extra_category_fields( $tag ) {
    $t_id = $tag->term_id;
    $cat_meta = get_option( "category_$t_id" );
?>
<tr class="form-field">
    <th scope="row" valign="top"><label for="meta-order"><?php _e('Category Order'); ?></label></th>
    <td>
        <div id="catOrder">
            <input type="text" name="cat_meta[order]" size="3" style="width:5%;" value="<?php echo (isset($cat_meta['order'])) ? $cat_meta['order'] : '0'; ?>" />
        </div>
        <span class="description"><?php _e('Categories are ordered Smallest to Largest'); ?></span>
    </td> 
</tr>
<?php
}
add_action('protax_edit_form_fields','extra_category_fields');  

/** Save Category Meta **/
function save_extra_category_fileds( $term_id ) {
    global $wpdb;
    if ( isset( $_POST['cat_meta'] ) ) {
        $t_id = $term_id;
        $cat_meta = get_option( "category_$t_id");
        $cat_keys = array_keys($_POST['cat_meta']);
            foreach ($cat_keys as $key){
            if (isset($_POST['cat_meta'][$key])){
                $cat_meta[$key] = $_POST['cat_meta'][$key];
            }
        }
        update_option( "category_$t_id", $cat_meta );
        $wpdb->update($wpdb->terms, array('term_group' => $cat_meta['order']), array('term_id'=>$term_id));
    }
}
add_action ( 'edited_protax', 'save_extra_category_fileds');

Calling it:

$procats = get_categories(array('taxonomy' => 'protax', 'hide_empty' => 0, 'orderby' => 'term_group'));

Edit - You're also able to order them in the admin panel by using this filter:

function change_term_order( $orderby, $args, $taxonomies ) {
    if ( is_admin() && 'protax' !== $taxonomies[0] )
        return $orderby;

    $orderby = 'term_group';
    $args['order'] = 'ASC';
    return $orderby;
}
add_filter( 'get_terms_orderby', 'change_term_order', 10, 3 );