Enable a node's menu item when on that node? /3, renewed

// Answer for question on Drupal Stack Exchange:
// http://drupal.stackexchange.com/questions/27907/enable-a-nodes-menu-item-when-on-that-node

// this is the template.php file!!
// ....

/**
 * @see http://api.drupal.org/api/drupal/includes!theme.inc/function/theme_link/7
 */
function YOURTHEMENAME_link($variables) {
    // in this example, I let "contact" and "node/9" links only be visible if the user is on these pages
    // and I let some other paths to be displayed at, see below...
    $your_paths_to_hide_array = array('contact', 'node/9'); // SUBSTITUTE IT with yours - empty cache every time you change that
    // these links will be visible only on their own pages -
    // but here you can define other paths where they can be visible at
    // if you don't want any other pages for these to be visible at, then
    // leave it empty (like this: array()),
    $link_also_visible_on_paths = array('node/3', 'any_other_path'); // SUBSTITUTE IT with your paths! - empty cache every time you change that
    $is_link_to_hide = in_array($variables['path'], $your_paths_to_hide_array);
    if ($is_link_to_hide) {
        $is_current_page_self = ($variables['path'] == $_GET['q']);
        $element_can_be_visible = $is_current_page_self || (!empty($link_also_visible_on_paths) && in_array($_GET['q'], $link_also_visible_on_paths));

        if (!$element_can_be_visible) {
            return '';
        }
    }
    return '<a href="' . check_plain(url($variables['path'], $variables['options'])) . '"' . drupal_attributes($variables['options']['attributes']) . '>' . ($variables['options']['html'] ? $variables['text'] : check_plain($variables['text'])) . '</a>';
}

// ....