PHPクラス初期化関数コードの説明

<!--?php
    class ShopProduct {
        public $title = "default product";
        public $producerMainName = "main name";
        public $producerFirstName = "first name";
        public $price = 0;
        function getProducer() {
            return "{$this--->producerFirstName}" . " {$this->producerMainName}";
        }
    $product1 = new ShopProduct();
    $product1->title = "My Antonia";
    $product1->producerMainName = "Cather";
    $product1->producerFirstName = "Willa";
    $product1->price = 5.99;
    print "author: {$product1->getProducer()}";
>

<!--?php  
    class ShopProduct {
        public $title;
        public $producerMainName;
        public $producerFirstName;
        public $price = 0;
        function __construct($title,$firstName,$mainName,$price) { //构造方法
            $this--->title = $title;
            $this->producerFirstName = $firstName;
            $this->producerMainName = $mainName;
            $this->price = $price;
        }
        function getProducer() {
            return "{$this->producerFirstName}" . " {$this->producerMainName}";
        }
    }
    $product1 = new ShopProduct( "My Antionia", "willa", "Cather", 5.99);
    print "author: {$product1->getProducer()}";
>