wp-adminbar toggler. Toggle adminbar from a URL parameter.

wp-adminbar-toggler.php

<?php

/**
 * wpadminbar toggle for conveniently showing/hiding adminbar from a URL parameter.
 *
 * USAGE:
 *   1. Upload wp-adminbar-toggler.php to theme
 *   2. Include this file in theme's functions.php
 *      - require_once( get_stylesheet_directory() . '/wp-adminbar-toggler.php' );
 *   3. Use ?adminbar=1 in the URL to show adminbar. Use ?adminbar=0 in the URL to 
 *      hide adminbar (this will persist the adminbar show/hide status using $_SESSION)
 * 
 * @return void
 */

add_filter('show_admin_bar', function($show_admin_bar) {

  if ( session_status() === PHP_SESSION_NONE ) @session_start();

  $adminbar_toggle = isset($_GET['adminbar']) ? (int) $_GET['adminbar'] : null;

  if ( $adminbar_toggle !== null && $adminbar_toggle === 1 ) {

    $_SESSION['adminbar'] = 1;

  } else if ( $adminbar_toggle !== null && $adminbar_toggle === 0 ) {

    $_SESSION['adminbar'] = 0;

  }

  $adminbar = isset($_SESSION['adminbar']) ? $_SESSION['adminbar'] : false;
  $adminbar = $adminbar === false ? (isset($_GET['adminbar']) && (int) $_GET['adminbar'] === 1 ? true : false) : $adminbar;

  /**
   * adminbar toggler has not beign triggered yet so don't do anything
   */
  if ( !isset($_SESSION['adminbar']) ) return $show_admin_bar;

  return $adminbar;

});