Case analysis of advanced features of PHP objects, patterns and practices

<?php
class StaticExample{
  static public $aNum = 10;
  static public function sayHello(){
    print "hello";
  }
}
print StaticExample::$aNum."<br/>";
StaticExample::sayHello();

<?php
class ShopProduct{
  private $title;
  private $producerMainName;
  private $producerFirstName;
  protected $price;
  private $discount = 0;
  private $id = 0;
  function __construct($title,$firstName,$mainName,$price){
    $this->title = $title;
    $this->producerFirstName = $firstName;
    $this->producerMainName = $mainName;
    $this->price = $price;
  }
  public function setID($id){
    $this->id = $id;
  }
  public static function getInstance($id,PDO $pdo){
    $query = "select * from products where id= &#39;$id&#39;";
    $stmt = $pdo->query($query);
    $row = $stmt->fetch();
    if(empty($row)){
      return null;
    }
    if($row[&#39;type&#39;] == "book"){
      $product = new BookProduct($row[&#39;title&#39;],
        $row[&#39;firstname&#39;],
        $row[&#39;mainname&#39;],
        $row[&#39;price&#39;],
        $row[&#39;numpages&#39;]
        );
    }else if($row[&#39;type&#39;] == "cd"){
      $product = new CdProduct($row[&#39;title&#39;],
        $row[&#39;firstname&#39;],
        $row[&#39;mainname&#39;],
        $row[&#39;price&#39;],
        $row[&#39;playLength&#39;]
        );
    }else{
      $product = new ShopProduct($row[&#39;title&#39;],
        $row[&#39;firstname&#39;],
        $row[&#39;mainname&#39;],
        $row[&#39;price&#39;]
        );
    }
    $product->setId($row[&#39;id&#39;]);
    $product->setDiscount($row[&#39;discount&#39;]);
    return $product;
  }
  public function getProducerFirstName(){
    return $this->producerFirstName;
  }
  public function getProducerMainName(){
    return $this->producerMainName;
  }
  public function setDiscount($num){
    $this->discount = $num;
  }
  public function getDiscount(){
    return $this->discount;
  }
  public function getTitle(){
    return $this->title;
  }
  public function getPrice(){
    return ($this->price - $this->discount);
  }
  function getProducer(){
    return $this->producerFirstName." ".$this->producerMainName;
  }
  function getSummaryLine(){
    $base = "$this->title({$this->producerMainName},";
    $base .= "{$this->producerFirstName})";
    return $base;
  }
}
class CdProduct extends ShopProduct{
  private $playLength;
  function __construct($title,$firstName,$mainName,$price,$playLength){
    parent::__construct($title,$firstName,$mainName,$price);//继承父类的构造函数
    $this->playLength = $playLength;
  }
  function getPlayLength(){
    return $this->playLength;
  }
  function getSummaryLine(){
    $base = parent::getSummaryLine();
    $base .= ":playing time {$this->playLength}";
    return $base;
  }
}
class BookProduct extends ShopProduct{
  private $numPages = 0;
  function __construct($title,$firstName,$mainName,$price,$numPages){
    parent::__construct($title,$firstName,$mainName,$price);
    $this->numPages = $numPages;
  }
  function getnumPages(){
    return $this->numPages;
  }
  function getSummaryLine(){
    $base = parent::getSummaryLine();
    $base .= ":page count {$this->numPages}";
    return $base;
  }
}
$dsn = "sqlite:C:/Users/Administrator/Desktop/shop.db";
$pdo = new PDO($dsn,null,null);
$pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
$obj = ShopProduct::getInstance(1,$pdo);
echo $obj->getSummaryLine();