PHP构造函数引用变量以使其他变量不起作用

我有一个构造函数。除了最后一部分，一切都正常

class Article {
  public $category;
  public $title;
  public $text;
  public $intro;


  public function __construct($category, $title, $text) {
      $this->category = $category;
      $this->title = $title;    
      $this->text = $text;
      $this->intro = substr($this->text, 0, 40);
  }
}

---

没有创建简介。

它工作正常：

<?php

class Article {
  public $category;
  public $title;
  public $text;
  public $intro;


  public function __construct($category, $title, $text) {
      $this->category = $category;
      $this->title = $title;    
      $this->text = $text;
      $this->intro = substr($this->text, 0, 40);
  }
}

$test=new Article('a','b','A question is a linguistic expression used to make a request for information, or the request made using such an expression. The information requested may be provided in the form of an answer.');

echo $test->intro; //A question is a linguistic expression us

---

哪一个是“最后一部分”以及它是如何不起作用的？请不要让免费帮助猜测Intro没有被创建。你能用不起作用的代码编辑吗？