php 類型指示

<?php
    class MyClass{
        public function test(OtherClass $otherclass){
            echo $otherclass->var;
        }
    }

    class OtherClass{
        public $var = 'Hello, World!';
    }

    $obj = new MyClass();
    $other = new OtherClass();

    $obj->test($other);
    $obj->test(2);     #爲什麼沒有報錯呢？
?>