PHP中的public、private和protected有什么区别？

public、private和protected称为访问修饰符。与C++一样, PHP也具有三个访问修饰符, 例如public, private和protected。属性, 方法或常量的可见性可以通过在声明的前面加上这些关键字来定义。

• 如果类成员声明为public, 则可以在任何地方访问它。
• 如果将类成员声明为protected, 则只能在类本身内部以及通过继承和父类访问它。
• 如果类成员声明为private, 那么只有定义该成员的类才能访问它。

public访问修饰符：

该修饰符可以在类的内部和外部使用。

例子：

<?php
  
//BaseClass
class pub {
     public $tag_line = "A Computer Science Portal for Geeks!" ;
     function display() {
         echo $this ->tag_line. "<br/>" ;
     }
}
  
//SubClass
class child extends pub {
     function show(){
         echo $this ->tag_line;
     }
} 
  
//Object Declaration
$obj = new child;
  
//A Computer Science Portal for Geeks!
echo $obj ->tag_line. "<br/>" ; 
  
//A Computer Science Portal for Geeks!
$obj ->display(); 
  
//A Computer Science Portal for Geeks!
$obj ->show(); 
?>

输出如下：

A Computer Science Portal for Geeks! A Computer Science Portal for Geeks! A Computer Science Portal for Geeks!

protected访问修饰符：该修饰符可以在定义它的类及其父类或继承的类中使用。

例子：

<?php
  
//Base Class
class pro {
     protected $x = 500;
     protected $y = 500;
              
     //Subtraction Function
     function sub() 
     {
         echo $sum = $this ->x- $this ->y . "<br/>" ;
     }     
} 
  
//SubClass - Inherited Class
class child extends pro {
     function mul() //Multiply Function
     {
         echo $sub = $this ->x* $this ->y;
     }
} 
  
$obj = new child;
$obj ->sub();
$obj ->mul();
?>

输出如下：

0 250000

private访问修饰符：该修饰符可以在定义它的类中使用。 (无法在继承的类中的类外部访问)。

例子：

<?php
  
//Base Class
class demo {
     private $name = "A Computer Science Portal for Geeks!" ;
      
     private function show()
     {
         echo "This is private method of base class" ;
     }
} 
  
//Sub Class
class child extends demo {
     function display()
     {
         echo $this ->name;
     }
} 
  
//Object Declaration
$obj = new child;
  
//Uncaught Error: Call to private method demo::show()
$obj ->show(); 
  
//Undefined property: child::$name
$obj ->display(); 
?>

输出如下：

It will display error because private class data can not be accessed outside the class

其他示例：

例子：

<?php
class BaseClass
{
     public $public = 'Public' ;
     protected $protected = 'Protected' ;
     private $private = 'Private' ;
  
     function Display()
     {
         echo $this -> public ;
         echo $this -> protected ;
         echo $this -> private ;
     }
}
  
$obj = new BaseClass();
echo $obj -> public ; 
echo $obj -> protected ; //Cannot access protected property
echo $obj -> private ; //Cannot access private property
$obj ->Display();  //Displays all properties
  
class SubClass extends BaseClass
{
     public $public = 'Public Sub Class' ;
     protected $protected = 'Protected Sub Class' ;
  
     function Display()
     {
         echo $this -> public ;
         echo $this -> protected ;
         echo $this -> private ;
     }
}
  
$obj2 = new SubClass();
echo $obj2 -> public ; 
echo $obj2 -> protected ; //Cannot access protected property
echo $obj2 -> private ;  //Cannot access priavte property
$obj2 ->Display(); //Displays all properties
?>

输出如下：

It will display error because private class data can not be accessed outside the class