PHPの__toString()メソッドの詳しい説明

<?php
class Person
{
    public $sex;
    public $name;
    public $age;
    public function __construct($name="",  $age=25, $sex=&#39;男&#39;)
    {
        $this->name = $name;
        $this->age  = $age;
        $this->sex  = $sex;
    }
    public function __toString()
    {
        return  &#39;go go go&#39;;
    }
}
$person = new Person(&#39;小明&#39;); // 初始赋值
echo $person;

go go go

<?php
class Person
{
    public $sex;
    public $name;
    public $age;
    public function __construct($name="",  $age=25, $sex=&#39;男&#39;)
    {
        $this->name = $name;
        $this->age  = $age;
        $this->sex  = $sex;
    }
    
}
$person = new Person(&#39;小明&#39;); // 初始赋值
echo $person;

Catchable fatal error: Object of class Person could not be converted to string in D:\phpStudy\WWW\test\index.php on line 18
很明显，页面报了一个致命错误，这是语法所不允许的。