8.接口水平扩展

如果父类太多，则会造成垂直方向继承过多，导致类的层级过深，从而难以维护，此时可以使用水平扩展代替垂直扩展。
实例如下：

interface Power
{
    public function power();
}
class Gas implements Power
{
    public function power()
    {
        return "汽油";
    }
}
class Battery implements Power
{
    public function power()
    {
        return "电池";
    }
}

<?php
interface CarContract
{
    public function drive();
}
abstract class BaseCar implements CarContract
{
    protected $brand;
    /**
     * @var Power
     */
    protected $power;
    public function __construct(Power $power, $brand)
    {
        $this->power = $power;
        $this->brand = $brand;
    }
    abstract public function drive();
}
class LynkCo01 extends BaseCar
{
    public function __construct(Power $power)
    {
        parent::__construct($power, '领克01');
    }
    public function drive()
    {
        echo "启动{$this->brand}汽车" . PHP_EOL;
        echo "动力来源: " . $this->power->power() . PHP_EOL;
    }
}