实例化没有显式构造函数的类是否合法？

I recently created two code examples to test how constructors work in php, one with an explicit constructor and other without. Both output was the same.

Is it legal to instantiate a class which has no explicit constructor?

Code with constructor

<?php
class Circle {
      const PI = 3.14;
      public $radius;

//Constructor method 
       public function __construct($r) {
             $this->radius = $r;
       }

       public function calculate() {
             return 2 * self::PI * $this->radius;
       }
}

//Argument passed during instantiating
 $C = new Circle(7);
 echo ("Circumference of Circle: ".$C->calculate());
?>

Code without Constructor

 <?php
   class Circle {
        const PI = 3.14;
        public $radius;

    //Non-ideal class mutation method 
            public function calculate($r) {
                $this->radius = $r;
                return 2 * self::PI * $this->radius;
            }
        }

     $C = new Circle();
    //Argument was passed directly to the method
    echo ("Circumference of Circle: ".$C->calculate(7));
 ?>

Adding a constructor method is optional. The documentation states:

PHP 5 allows developers to declare constructor methods for classes. Classes which have a constructor method call this method on each newly-created object.

Note however that your second implementation is not ideal:

• the calculate method has a side-effect: it sets the radius member. This is not best practice: if a method's main purpose is to return something, then it should not mutate the object.

• the calculate method requires you to provide the radius on every call. This is not really what you want when the object is supposed to know its radius.