参数和类型

PHP是一种弱类型语言，即变量不需要声明为特定的数据类型，这既是一种便利，也是一种麻烦（尤其是当你希望一个参数必须是某种类型的时候）。

为了解决这个问题，PHP5引入了类型提示。要增加一个方法参数的类型提示，只需简单地将类名放在需要约束的方法参数之前。

<?php

// 类型提示
class ShopProduct {
    private $title        = '商品名称';
    private $producerName = '制造商名称';
    private $place        = '产地';
    protected $price      = 0;
    private $discount     = 0;

    function __construct( $title, $producerName, $place, $price ) {
        $this->title        = $title;
        $this->producerName = $producerName;
        $this ->place       = $place;
        $this->price        = $price;
    }
    function getTitle() {
        return $this->title;
    }
    function getProducerName() {
        return $this->producerName;
    }
    function getPlace() {
        return $this->place;
    }
    function setDiscount( $num ) {
        $this->discount = $num;
    }
    function getDiscount() {
        return $this->discount;
    }
    function getPrice() {
        return $this->price - $this->price*$this->discount;
    }
    function getProducer() {
        return "$this->place" . "$this->producerName";
    }
    function getSummaryLine() {
        $base = "$this->title" . "\t" . "$this->place" . "\t" . "$this->producerName";
        return $base;
    }
}


/**
 * 商品信息输出类
 */
class ShopProductWriter {
    public function write( $shopProduct ) {
        $str = $shopProduct->getTitle() . ":" . $shopProduct->getProducer() . $shopProduct->getPrice() . "\n";
        print $str;
    }
}

// 测试输出类
$product1 = new ShopProduct( "《洗脑术》", '中国青年出版社', '北京', 32 );
$writer   = new ShopProductWriter();
$writer->write( $product1 );

//虽然我们把ShopProductWriter中的write方法的参数写为$ShopProduct，但实际上它可以接受非预期的数据类型，
// 而方法内部也是按照接受的参数为$ShopProduct对象来处理的。
// 为了避免传入非预期对象，我们可以使用PHP提供的类型提示来达到只接受特定数据类型的目的
// （只能限定对象、数组、和NULL，其他数据类型限定暂不支持）

/**
 * 增加了类型提示的write方法
 */
class SPWriter {
    function write( ShopProduct $shopProduct ) {
        $str = "商品名称：" . $shopProduct->getTitle() . "\t" .
            "生产商：" . $shopProduct->getProducer() . ":\t" .
            "价格：" . $shopProduct->getPrice() . "\n";
        print $str;
    }
}

$product1 = new ShopProduct( "《洗脑术》", '中国青年出版社', '北京', 32 );
$writer = new SPWriter();
// 参数类型正确
$writer->write( $product1 );
// 参数类型不正确，会输出错误信息
$writer->write( '测试字符' );
?>

类型提示不能用于规定参数为某种基本数据类型（如字符串、整型等等）。