为什么我不能直接从它自己的类中访问受保护的属性？

Question

我是 PHP 新手。我现在学习可见性范围概念（也称为访问修饰符）。

我也在这个论坛上阅读了以下两个链接：

Failed to get an object property that containing ":protected"

What is the difference between public, private, and protected?

我在名为“class.Address.inc”的文件中创建了一个简单的类：

<?php

/**
 * Physical address. 
 */
class Address {
  // Street address.
  public $street_address_1;
  public $street_address_2;

  // Name of the City.
  public $city_name;

  // Name of the subdivison.
  public $subdivision_name;

  // Postal code.
  public $postal_code;

  // Name of the Country.
  public $country_name;

  // Primary key of an Address.
  protected $_address_id;

  // When the record was created and last updated.
  protected $_time_created;
  protected $_time_updated;

  /**
   * Display an address in HTML.
   * @return string 
   */
  function display() {
    $output = '';

    // Street address.
    $output .= $this->street_address_1;
    if ($this->street_address_2) {
      $output .= '<br/>' . $this->street_address_2;
    }

    // City, Subdivision Postal.
    $output .= '<br/>';
    $output .= $this->city_name . ', ' . $this->subdivision_name;
    $output .= ' ' . $this->postal_code;

    // Country.
    $output .= '<br/>';
    $output .= $this->country_name;

    return $output;
  }
}

然后，我在文件demo.php中创建了一个简单的程序如下：

require 'class.Address.inc';

echo '<h2>Instantiating Address</h2>';
$address = new Address;

echo '<h2>Empty Address</h2>';
echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';

echo '<h2>Setting properties...</h2>';
$address->street_address_1 = '555 Fake Street';
$address->city_name = 'Townsville';
$address->subdivision_name = 'State';
$address->postal_code = '12345';
$address->country_name = 'United States of America';
echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';

echo '<h2>Displaying address...</h2>';
echo $address->display();

echo '<h2>Testing protected access.</h2>';
echo "Address ID: {$address->_address_id}";

除了最后一行之外，上述程序中的一切都有效。我收到一个致命错误，说我无法访问“_address_id 属性”。为什么？

受保护的范围是当您想让您的变量/函数在所有扩展当前类（包括父类）的类中可见时。

"$address" 对象来自当前名为 Address 的类。那我做错了什么？

请帮忙。

Qwerty

Answer 1

尝试访问受保护属性的代码必须位于类的方法或扩展它的类中。您询问的 echo 行不在任何类方法中，它在脚本的全局代码中。受保护的和私有的属性在类之外是不可见的。