PHP: 访问控制（可见性）

访问控制（可见性）

对属性或方法的访问控制（PHP 7.1.0 以后支持常量），是通过在前面添加关键字 public（公有），protected（受保护）或 private（私有）来实现的。被定义为公有的类成员可以在任何地方被访问。被定义为受保护的类成员则可以被其自身以及其子类和父类访问。被定义为私有的类成员则只能被其定义所在的类访问。

属性的访问控制

类属性可以定义为public， private 或者 protected。在没有任何访问控制关键字的情况下，属性声明为 public。

示例 #1 属性声明

<?php
/**
 * Define MyClass
 */
class MyClass
{
    public $public = 'Public';
    protected $protected = 'Protected';
    private $private = 'Private';

    function printHello()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj = new MyClass();
echo $obj->public; // 这行能被正常执行
echo $obj->protected; // 这行会产生一个致命错误
echo $obj->private; // 这行也会产生一个致命错误
$obj->printHello(); // 输出 Public、Protected 和 Private


/**
 * Define MyClass2
 */
class MyClass2 extends MyClass
{
    // 可以对 public 和 protected 进行重定义，但 private 而不能
    public $public = 'Public2';
    protected $protected = 'Protected2';

    function printHello()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj2 = new MyClass2();
echo $obj2->public; // 这行能被正常执行
echo $obj2->protected; // 这行会产生一个致命错误
echo $obj2->private; // 未定义 private
$obj2->printHello(); // 输出 Public2、Protected2 和 Undefined

?>

方法的访问控制

类中的方法可以被定义为 public、private 或 protected。如果没有设置这些关键字，则该方法默认为 public。

示例 #2 方法声明

<?php
/**
 * Define MyClass
 */
class MyClass
{
    // 声明一个公有的构造函数
    public function __construct() { }

    // 声明一个公有的方法
    public function MyPublic() { }

    // 声明一个受保护的方法
    protected function MyProtected() { }

    // 声明一个私有的方法
    private function MyPrivate() { }

    // 此方法为公有
    function Foo()
    {
        $this->MyPublic();
        $this->MyProtected();
        $this->MyPrivate();
    }
}

$myclass = new MyClass;
$myclass->MyPublic(); // 这行能被正常执行
$myclass->MyProtected(); // 这行会产生一个致命错误
$myclass->MyPrivate(); // 这行会产生一个致命错误
$myclass->Foo(); // 公有，受保护，私有都可以执行


/**
 * Define MyClass2
 */
class MyClass2 extends MyClass
{
    // 此方法为公有
    function Foo2()
    {
        $this->MyPublic();
        $this->MyProtected();
        $this->MyPrivate(); // 这行会产生一个致命错误
    }
}

$myclass2 = new MyClass2;
$myclass2->MyPublic(); // 这行能被正常执行
$myclass2->Foo2(); // 公有的和受保护的都可执行，但私有的不行

class Bar 
{
    public function test() {
        $this->testPrivate();
        $this->testPublic();
    }

    public function testPublic() {
        echo "Bar::testPublic\n";
    }
    
    private function testPrivate() {
        echo "Bar::testPrivate\n";
    }
}

class Foo extends Bar 
{
    public function testPublic() {
        echo "Foo::testPublic\n";
    }
    
    private function testPrivate() {
        echo "Foo::testPrivate\n";
    }
}

$myFoo = new foo();
$myFoo->test(); // Bar::testPrivate 
                // Foo::testPublic
?>

常量的控制访问

PHP 7.1.0 开始，类的常量可以定义为 public、private 或 protected。如果没有设置这些关键字，则该常量默认为 public。

示例 #3 PHP 7.1.0 中的常量声明

<?php
/**
 * Define MyClass
 */
class MyClass
{
    // 公有常量
    public const MY_PUBLIC = 'public';

    // 受保护的常量
    protected const MY_PROTECTED = 'protected';

    // 私有常量
    private const MY_PRIVATE = 'private';

    public function foo()
    {
        echo self::MY_PUBLIC;
        echo self::MY_PROTECTED;
        echo self::MY_PRIVATE;
    }
}

$myclass = new MyClass();
MyClass::MY_PUBLIC; // 这行可以正常执行
MyClass::MY_PROTECTED; // 这行会产生一个致命错误 
MyClass::MY_PRIVATE; // 这行会产生一个致命错误
$myclass->foo(); // 将会输出：Public Protected Private


/**
 * Define MyClass2
 */
class MyClass2 extends MyClass
{
    // This is public
    function foo2()
    {
        echo self::MY_PUBLIC;
        echo self::MY_PROTECTED;
        echo self::MY_PRIVATE; // 这行会产生一个致命错误
    }
}

$myclass2 = new MyClass2;
echo MyClass2::MY_PUBLIC; // 这行可以正常执行
$myclass2->foo2(); // 将会输出：Public Protected，MY_PRIVATE 是私有常量，无法输出
?>

其它对象的访问控制

同一个类的对象即使不是同一个实例也可以互相访问对方的 private 与 protected 成员。这是由于在这些对象的内部具体实现的细节都是已知的。

示例 #4 访问同一个对象类型的 private 成员

<?php
class Test
{
    private $foo;

    public function __construct($foo)
    {
        $this->foo = $foo;
    }

    private function bar()
    {
        echo 'Accessed the private method.';
    }

    public function baz(Test $other)
    {
        // 我们可以改变 private 属性：
        $other->foo = 'hello';
        var_dump($other->foo);

        // 我们也可以调用 private 方法：
        $other->bar();
    }
}

$test = new Test('test');

$test->baz(new Test('other'));
?>

以上示例会输出：

string(5) "hello"
Accessed the private method.

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User Contributed Notes 7 notes

down

pgl at yoyo dot org ¶

9 years ago


Just a quick note that it's possible to declare visibility for multiple properties at the same time, by separating them by commas.

eg:

<?php
class a
{
    protected $a, $b;

    public $c, $d;

    private $e, $f;
}
?>

down

r dot wilczek at web-appz dot de ¶

18 years ago


Beware: Visibility works on a per-class-base and does not prevent instances of the same class accessing each others properties!

<?php
class Foo
{
    private $bar;

    public function debugBar(Foo $object)
    {
        // this does NOT violate visibility although $bar is private
        echo $object->bar, "\n";
    }

    public function setBar($value)
    {
        // Neccessary method, for $bar is invisible outside the class
        $this->bar = $value;
    }
    
    public function setForeignBar(Foo $object, $value)
    {
        // this does NOT violate visibility!
        $object->bar = $value;
    }
}

$a = new Foo();
$b = new Foo();
$a->setBar(1);
$b->setBar(2);
$a->debugBar($b);        // 2
$b->debugBar($a);        // 1
$a->setForeignBar($b, 3);
$b->setForeignBar($a, 4);
$a->debugBar($b);        // 3
$b->debugBar($a);        // 4
?>

down

alperenberatdurmus at gmail dot com ¶

1 year ago


Dynamic properties are "public".
<?php
class MyClass {
    public function setProperty($value) {
        $this->dynamicProperty = $value;
    }
}
$obj = new MyClass();
$obj->setProperty('Hello World');
echo $obj->dynamicProperty; // Outputs "Hello World"
?>

This usage is the same as well:
<?php
class MyClass {
}
$obj = new MyClass();
$obj->dynamicProperty = 'Hello World';
echo $obj->dynamicProperty; // Outputs "Hello World"
?>

down

jc dot flash at gmail dot com ¶

12 years ago


if not overwritten, self::$foo in a subclass actually refers to parent's self::$foo 
<?php
class one
{
    protected static $foo = "bar";
    public function change_foo($value)
    {
        self::$foo = $value;
    }
}

class two extends one
{
    public function tell_me()
    {
        echo self::$foo;
    }
}
$first = new one;
$second = new two;

$second->tell_me(); // bar
$first->change_foo("restaurant");
$second->tell_me(); // restaurant
?>

down

Joshua Watt ¶

17 years ago


I couldn't find this documented anywhere, but you can access protected and private member varaibles in different instance of the same class, just as you would expect

i.e.

<?php
class A
{
    protected $prot;
    private $priv;
    
    public function __construct($a, $b)
    {
        $this->prot = $a;
        $this->priv = $b;
    }
    
    public function print_other(A $other)
    {
        echo $other->prot;
        echo $other->priv;
    }
}

class B extends A
{
}

$a = new A("a_protected", "a_private");
$other_a = new A("other_a_protected", "other_a_private");

$b = new B("b_protected", "ba_private");

$other_a->print_other($a); //echoes a_protected and a_private
$other_a->print_other($b); //echoes b_protected and ba_private

$b->print_other($a); //echoes a_protected and a_private
?>

down

bishop at php dot net ¶

7 years ago


> Members declared protected can be accessed only within 
> the class itself and by inherited classes. Members declared 
> as private may only be accessed by the class that defines 
> the member.

This is not strictly true. Code outside the object can get and set private and protected members:

<?php
class Sealed { private $value = 'foo'; }

$sealed = new Sealed;
var_dump($sealed); // private $value => string(3) "foo"

call_user_func(\Closure::bind(
    function () use ($sealed) { $sealed->value = 'BAZ'; },
    null,
    $sealed
));

var_dump($sealed); // private $value => string(3) "BAZ"

?>

The magic lay in \Closure::bind, which allows an anonymous function to bind to a particular class scope. The documentation on \Closure::bind says:

> If an object is given, the type of the object will be used
> instead. This determines the visibility of protected and
> private methods of the bound object.

So, effectively, we're adding a run-time setter to $sealed, then calling that setter. This can be elaborated to generic functions that can force set and force get object members:

<?php
function force_set($object, $property, $value) {
    call_user_func(\Closure::bind(
        function () use ($object, $property, $value) {
            $object->{$property} = $value;
        },
        null,
        $object
    ));
}

function force_get($object, $property) {
    return call_user_func(\Closure::bind(
        function () use ($object, $property) {
            return $object->{$property};
        },
        null,
        $object
    ));
}

force_set($sealed, 'value', 'quux');
var_dump(force_get($sealed, 'value')); // 'quux'

?>

You should probably not rely on this ability for production quality code, but having this ability for debugging and testing is handy.

down

kostya at eltexsoft dot com ¶

3 years ago


I see we can redeclare private properties into child class 
<?php   
 class A{
        private int $private_prop = 4;
        protected int $protected_prop = 8;
    }

    class B extends A{
        private int $private_prop = 7; // we can redeclare private property!!!
        public function printAll() {
            echo $this->private_prop;
            echo $this->protected_prop;
    }
    }

    $b = new B;
    $b->printAll(); // show 78
}
?>

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