在构造函数设置之前没有实例化的类属性 - 不好吗？

Question

偶然地，我注意到以下内容适用于PHP：

class
{
    public function __construct()
    {
        // Notice that this was not declared before use
        $this->not_declared = "Hello";
    }

    public function display()
    {
        echo $this->not_declared; // Outputs "Hello"
    }
}

这是不好的做法吗？我总是认为必须在使用前声明属性，如下所示：

class
{
    private $not_declared = "";

    public function __construct()
    {
        // In this case, it was declared above
        $this->not_declared = "Hello";
    }

    public function display()
    {
        echo $this->not_declared; // Outputs "Hello"
    }
}