PHP多类继承

Question

我有一个情况我有三类：

<?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. 
?> 

我在做什么错？ PHP支持多级继承...我错过了什么？看起来像C里面可以访问父类，没问题，但变量$ obj不能。 $ obj是否能够访问A的继承内容（受保护）？注意：使用PHP v5.6.25

Answer 1

尝试以下操作：

<?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); 

你用来编写代码的语法是完全错误的。定义一个方法，正确的方法是：

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;); 

同样在PHP中的所有变量应$像$a_val不a_val启动。并且protected属性在类及其子类之外不可用。

看一看这些页面：Visibility，A simple tutorial和OOP Basics