php中使用接口的知识，定义一个形状，该形状包含面积计算公式和周长计算公式；分别定义圆形，矩形，三角形，都继承于该形状，都进行具体的面积和周长计算；

好的，以下是实现代码：

// 定义形状接口
interface Shape {
  public function area();
  public function perimeter();
}

// 定义圆形类
class Circle implements Shape {
  private $radius;

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

  public function area() {
    return pi() * $this->radius * $this->radius;
  }

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

// 定义矩形类
class Rectangle implements Shape {
  private $length;
  private $width;

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

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

  public function perimeter() {
    return 2 * ($this->length + $this->width);
  }
}

// 定义三角形类
class Triangle implements Shape {
  private $a;
  private $b;
  private $c;

  public function __construct($a, $b, $c) {
    $this->a = $a;
    $this->b = $b;
    $this->c = $c;
  }

  public function area() {
    $s = ($this->a + $this->b + $this->c) / 2;
    return sqrt($s * ($s - $this->a) * ($s - $this->b) * ($s - $this->c));
  }

  public function perimeter() {
    return $this->a + $this->b + $this->c;
  }
}

使用示例：

// 创建圆形对象
$circle = new Circle(5);
echo "圆形的面积为：" . $circle->area() . "\n";
echo "圆形的周长为：" . $circle->perimeter() . "\n";

// 创建矩形对象
$rectangle = new Rectangle(4, 6);
echo "矩形的面积为：" . $rectangle->area() . "\n";
echo "矩形的周长为：" . $rectangle->perimeter() . "\n";

// 创建三角形对象
$triangle = new Triangle(3, 4, 5);
echo "三角形的面积为：" . $triangle->area() . "\n";
echo "三角形的周长为：" . $triangle->perimeter() . "\n";

输出结果：

圆形的面积为：78.539816339745
圆形的周长为：31.415926535898
矩形的面积为：24
矩形的周长为：20
三角形的面积为：6
三角形的周长为：12