如何在PHP中使用多态和继承来处理数据类型

class Animal {
    protected $name;

    public function __construct($name) {
        $this->name = $name;
    }

    public function eat() {
        echo $this->name . " is eating." . PHP_EOL;
    }
}

class Cat extends Animal {
    public function meow() {
        echo $this->name . " is meowing." . PHP_EOL;
    }
}

$cat = new Cat("Tom");
$cat->eat();  // 输出：Tom is eating.
$cat->meow();  // 输出：Tom is meowing.

abstract class Shape {
    abstract public function calculateArea();
}

class Rectangle extends Shape {
    protected $length;
    protected $width;

    public function __construct($length, $width) {
        $this->length = $length;
        $this->width = $width;
    }

    public function calculateArea() {
        return $this->length * $this->width;
    }
}

class Circle extends Shape {
    protected $radius;

    public function __construct($radius) {
        $this->radius = $radius;
    }

    public function calculateArea() {
        return pi() * pow($this->radius, 2);
    }
}

function printArea(Shape $shape) {
    echo "The area is: " . $shape->calculateArea() . PHP_EOL;
}

$rectangle = new Rectangle(10, 5);
$circle = new Circle(7);

printArea($rectangle);  // 输出：The area is: 50
printArea($circle);  // 输出：The area is: 153.9380400259