Add metabox to Link Manager

tps-link-manager-metabox.php

<?php
/*
 * Add metabox to Link Manager
 * Description: Add attributes to each link, each link's attributes are saved serialized as an option.
 * Author: Tatiane Pires
 * Author URI: http://tatianepires.com.br
 * Version: 1.0
 */
 
/*
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 */

 /*
  * To enable Link Manager add to your functions.php file:
    add_filter( 'pre_option_link_manager_enabled', '__return_true' );
  */

add_action( 'add_meta_boxes', 'tps_link_metabox' );

/* Do something with the data entered */
add_action( 'add_link', 'tps_link_meta_update_postdata' );
add_action( 'edit_link', 'tps_link_meta_update_postdata' );

/* Adds a box to the mainside column on the Link Manager edit screen */
function tps_link_metabox() {
    $screens = array( 'link' );
    foreach ($screens as $screen) {
        add_meta_box(
            'tps_link_box',
            'Link Attributes',
            'tps_link_inner_box',
            $screen,
            'side',
            'core',
        );
    }
}

/* Prints the box content */
function tps_link_inner_box( $post ) {

    // Use nonce for verification
    wp_nonce_field( plugin_basename( __FILE__ ), 'tps_link_nonce' );

    $tps_link_field_option_values = tps_link_get_option(tps_link_get_id());
    $link_id = tps_link_get_id();

    $is_yes_checked = ($tps_link_field_option_values['title'] === tps_link_get_option_value('YES')) ? 'checked' : '';
    $is_no_checked = (!$is_yes_checked) ? 'checked' : '';

    $tps_link_field_option_yes = tps_link_get_option_value('YES');
    $tps_link_field_option_no = tps_link_get_option_value('NO');

    //$tps_link_field_option_order = isset($tps_link_field_option_values['order']) ? intval($tps_link_field_option_values['order']) : 99;
    $tps_link_field_option_order = $tps_link_field_option_values['order'];

    echo "<!-- ";
    echo "tps_link_field_option_values: ";
    var_dump($tps_link_field_option_values);
    echo " -->";

    echo <<<TPS_LINK_METABOX
    <div>
        <div class="inside" style="border-bottom: 1px solid #ccc;">
            <strong>Title Link</strong>
            <p>Display this link at the top of it's category's list?</p>
            <div>
                <input type="radio" id="tps_link_field_is_title_yes" name="tps_link_field_is_title" value="$tps_link_field_option_yes" $is_yes_checked />
                <label for="tps_link_field_is_title_yes">Yes</label>
            </div>
            <div>
                <input type="radio" id="tps_link_field_is_title_no" name="tps_link_field_is_title" value="$tps_link_field_option_no" $is_no_checked />
                <label for="tps_link_field_is_title_no">No</label>
            </div>
        </div>
        <div class="inside">
            <label for="tps_link_field_order"><strong>Link Order</strong>
            <p>Set the order to display the links inside the category.</p>
            <div>
                <input type="number" id="tps_link_field_order" name="tps_link_field_order" placeholder="0" value="$tps_link_field_option_order" />
            </div>
        </div>
    </div>
    TPS_LINK_METABOX;
}

function tps_link_meta_update_postdata( $link_id ) {
    // verify if this is an auto save routine. 
    // If it is our form has not been submitted, so we dont want to do anything
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
        return;

    // verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times

    if ( !isset( $_POST['tps_link_nonce'] ) || !wp_verify_nonce( $_POST['tps_link_nonce'], plugin_basename( __FILE__ ) ) )
        return;

    // OK, we're authenticated: we need to find and save the data

    //sanitize user input
    $tps_link_data = array(
        'title' => isset($_POST['tps_link_field_is_title']) ? sanitize_text_field($_POST['tps_link_field_is_title']) : $tps_link_field_options['NO'],
        'order' => isset($_POST['tps_link_field_order']) ? intval(sanitize_text_field($_POST['tps_link_field_order'])) : 0,
    );

    tps_link_update_option($link_id, $tps_link_data);
}

function tps_link_get_id() {
    $link_id = null;
    if (isset($_GET['link_id'])) {
        $link_id = $_GET['link_id'];
    } else {
        global $wpdb;
        $result  = $wpdb->get_results( "SHOW TABLE STATUS LIKE 'wp_links'", ARRAY_A );
        $link_id = $result[0]['Auto_increment'];
    }

    return $link_id;
}

function tps_link_option_name($link_id) {
    return sprintf('tps_link_options_%s', $link_id);
}

function tps_link_get_option($link_id) {
    $option = get_option(tps_link_option_name($link_id));

    return unserialize($option);
}

function tps_link_update_option($link_id, $link_data) {
    $tps_link_option_name = tps_link_option_name($link_id);
    $tps_link_option_value = tps_link_get_option($link_id);

    $serialized_data = serialize($link_data);

    if ($link_option_value === false) {
        add_option($tps_link_option_name, $serialized_data);  
    } else {
        update_option($tps_link_option_name, $serialized_data);
    }
}

function tps_link_get_option_value($option) {
    // Prevent that different values might be stored as this field's option
    $tps_link_field_options = array(
        'YES' => 'yes',
        'NO' => 'no',
    );

    return $tps_link_field_options[$option];
}