PHP多類繼承

Question

我有一個情況我有三類：

<?php 
class A { 
    protected a_val = 'some_val'; 
    public function read_a_val(return $this->a_val;); 
} 

class B extends A { 
    protected b_val = 'some_val_2'; 
    public function read_b_val(return $this->b_val;); 
} 

class C extends B { 
    protected c_val = 'some_val_3'; 
    public function read_c_val(return $this->c_val;); 
    public function c_read_a_val(return $this->a_val;); 
} 

$obj = new C; 
$val = $obj->read_a_val(); // <-- throws error; undefined method 
$val = $obj->a_val;   // <-- throws error; cannot access protected property 
$val = $obj->c_read_a_val(); // works. 
?> 

我在做什麼錯？ PHP支持多級繼承...我錯過了什麼？看起來像C裏面可以訪問父類，沒問題，但變量$ obj不能。 $ obj是否能夠訪問A的繼承內容（受保護）？注意：使用PHP v5.6.25

Answer 1

嘗試以下操作：

<?php 
class A { 
    protected $a_val = 'some_val'; 
    function read_a_val(){ return $this->a_val; } 
} 

class B extends A { 
    protected $b_val = 'some_val_2'; 
function read_b_val(){ return $this->b_val; } 
} 

class C extends B { 
    protected $c_val = 'some_val_3'; 
function read_c_val(){ return $this->c_val; } 
    function c_read_a_val(){ return $this->a_val; } 
} 

$obj = new C; 
$val1 = $obj->read_a_val(); 
$val2 = $obj->c_read_a_val(); 

var_dump($val1, $val2); 

你用來編寫代碼的語法是完全錯誤的。定義一個方法，正確的方法是：

function read_a_val(){ return $this->a_val; } 

不：

//See the curly braces in the above code and none here. 
public function read_a_val(return $this->a_val;); 

同樣在PHP中的所有變量應$像$a_val不a_val啓動。並且protected屬性在類及其子類之外不可用。

看一看這些頁面：Visibility，A simple tutorial和OOP Basics