Instantiating class and passing action - php

Question

I'm working through a PHP starter book I just got along with code. I am stuck on this one example and I have no idea what's going on. Can someone help explain what is going on?
This class seems to get instantiated and the default case is to list the posts recorded to a database:
<?php
require_once('../includes/admin.php');
$admin_posts = new Posts;
When the class gets called, the default case loads options to pass the id and action to the php file above, then it will parse and call another method. In my case, i'm trying to delete a post, but I keep getting an error that says object not found.
<?php
session_start();
require_once('database.php');
class Adminpanel{
public function __construct(){
$inactive = 600;
if (isset($_SESSION["kickstart_login"])) {
$sessionTTL = time() - $_SESSION["timeout"];
if ($sessionTTL > $inactive) {
session_unset();
session_destroy();
header("Location: http://".$_SERVER['SERVER_NAME']."/login.php?status=inactive");
}
}
$_SESSION["timeout"] = time();
$login = $_SESSION['kickstart_login'];
if(empty($login)){
session_unset();
session_destroy();
header('Location: http://'.$_SERVER['SERVER_NAME'].'/login.php?status=loggedout');
}else{
$this->ksdb = new Database;
$this->base = (object) '';
$this->base->url = 'http://'.$_SERVER['SERVER_NAME'];
}
}
}
class Posts extends Adminpanel{
public function __construct(){
parent::__construct();
if(!empty($_GET['action'])){
switch ($_GET['action']){
case 'create':
$this->addPost();
break;
default:
$this->listPosts();
break;
case 'save':
$this->savePost();
break;
case 'edit':
echo "string";
$this->editPosts();
break;
case 'delete':
$this->deletePost();
break;
}
}else{
$this->listPosts();
}
}
public function listPosts(){
$posts = $return = array();
$query = $this->ksdb->db->prepare("SELECT * FROM posts");
try {
$query->execute();
for($i=0; $row = $query->fetch(); $i++){
$return[$i] = array();
foreach($row as $key => $rowitem){
$return[$i][$key] = $rowitem;
}
}
}catch (PDOException $e) {
echo $e->getMessage();
}
$posts = $return;
require_once('templates/manageposts.php');
}
public function editPosts(){
echo "string";
}
public function addPost(){
require_once('templates/newpost.php');
}
public function savePost(){
$status= '';
$array = $format = $return = array();
if(!empty($_POST['post'])){
$post = $_POST['post'];
}
if(!empty($post['content'])){
$array['content'] = $post['content'];
$format[] = ':content';
}
if(!empty($post['title'])){
$array['title'] = $post['title'];
$format[] = ':title';
}
$cols = $values = '';
$i=0;
foreach($array as $col => $data){
if($i==0){
$cols .= $col;
$values .= $format[$i];
}else{
$cols .= ','.$col;
$values .= ','.$format[$i];
}
$i++;
}
try {
$query = $this->ksdb->db->prepare("INSERT INTO posts(".$cols.") VALUES (".$values.")");
for($c=0;$c<$i;$c++){
$query->bindParam($format[$c], ${'var'.$c});
}
$z=0;
foreach($array as $col => $data){
${'var' . $z} = $data;
$z++;
}
$result = $query->execute();
$add = $query->rowCount();
}catch (PDOException $e) {
echo $e->getMessage();
}
$query->closeCursor();
$this->db = null;
if(!empty($add) && $add > 0){
$status = array('success' => 'Your post has been saved successfully.');
$key = 'success';
}else{
$status = array('error' => 'There has been an error saving your post. Please try again later.');
$key = 'error';
}
header("Location: ".$this->base->url."/posts.php?status=".$key);
}
public function deletePost(){
echo "string";
// if(!empty($_GET['id']) && is_numeric($_GET['id'])){
// $query = "DELETE FROM `posts` WHERE id = ".$_GET['id'];
// $result = $this->db->query($query);
// $delete = $result->rowCount();
// $this->db = null;
// if(!empty($delete) && $delete > 0){
// header("Location: ".$this->base->url."/posts.php?delete=success");
// }else{
// header("Location: ".$this->base->url."/posts.php?delete=error");
// }
// }
}
}
class Comments extends Adminpanel{
public function __construct(){
parent::__construct();
if(!empty($_GET['action']) && $_GET['action'] == 'delete'){
$this->deleteComment();
}else{
$this->listComments();
}
}
public function listComments(){
$comments = $return = array();
$query = $this->ksdb->db->prepare("SELECT * FROM comments");
try {
$query->execute();
for($i=0; $row = $query->fetch(); $i++){
$return[$i] = array();
foreach($row as $key => $rowitem){
$return[$i][$key] = $rowitem;
}
}
}catch (PDOException $e) {
echo $e->getMessage();
}
$comments = $return;
require_once('templates/managecomments.php');
}
public function deleteComment(){
if(!empty($_GET['id']) && is_numeric($_GET['id'])){
$query = "DELETE FROM `comments` WHERE id = ".$_GET['id'];
$result = $this->db->query($query);
$delete = $result->rowCount();
$this->db = null;
if(!empty($delete) && $delete > 0){
header("Location: ".$this->base->url."/comments.php?delete=success");
}else{
header("Location: ".$this->base->url."/comments.php?delete=error");
}
}
}
}
$admin = new Adminpanel();