php - 一个类可以实例化另一个类吗？ (PHP)

Question

我试过这个，当我尝试在类“second”中实例化类“first”时出现错误。

类“second”中的注释部分导致错误。

class first {
    public $a;

    function __construct() {
        $this->a = 'a';
    }
}

class second {
    //$fst = new first();
    //public showfirst() {
        //$firsta = $this->first->a;
    //  echo "Here is first \$a: " . $firsta;
    //}
}

编辑:

这会导致服务器错误，即使我在类“second”中只有类“first”的实例化。

class second {
    $fst = new first();
    //public showfirsta() {
    //  $firsta = $this->fst->a;
    //  echo "Here is first \$a: " . $firsta;
    //}
}

Answer 1

试试这个:

class First {
    public $a;

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

    public function getA() {
      return $this->a;
    }
}

    class Second {
        protected $fst;
        public function __construct() {
          $this->fst = new First();
        }

        public function showfirst() {
           $firsta = $this->fst->getA();
           echo "Here is first {$firsta}";
        }
    }

    $test = new Second();
    $test->showfirst();