浅谈PHP的Public、Protected、Private三种方法的区别

public:权限是最大的，可以内部调用，实例调用等。
protected: 受保护类型，用于本类和继承类调用。
private: 私有类型，只有在本类中使用。

<?php
error_reporting(E_ALL);

class test{

         public $public;
         private $private;
         protected $protected;
         static $instance;

         public  function __construct(){
             $this->public    = 'public     <br>';
             $this->private   = 'private    <br>';
             $this->protected = 'protected  <br>';
         }

        static function tank(){
             if (!isset(self::$instance[get_class()]))
             {
                 $c = get_class();
                 self::$instance = new $c;
             }
             return self::$instance;
         }    

        public function pub_function() {
             echo "you request public function<br>";
             echo $this->public;
             echo $this->private;        //private,内部可以调用
             echo $this->protected;      //protected,内部可以调用
             $this->pri_function();      //private方法，内部可以调用
             $this->pro_function();      //protected方法，内部可以调用
         }

         protected  function pro_function(){
             echo "you request protected function<br>";
         }
         private function pri_function(){
             echo "your request private function<br>";
         }
    }


        $test = test::tank();
        echo $test->public;
        echo $test->private;    //Fatal error: Cannot access private property test::$private
        echo $test->protected;  //Fatal error: Cannot access protected property test::$protected
        
        $test->pub_function();
        $test->pro_function();  //Fatal error: Call to protected method test::pro_function() from context
        $test->pri_function();  //Fatal error: Call to private method test::pri_function() from context
        ?>

从上面的例子中，我们可以看出，
public:    可以class内部调用，可以实例化调用。
private:   可以class内部调用，实例化调用报错。
protected：  可以class内部调用，实例化调用报错。

---

<?php
class test{

    public $public;
    private $private;
    protected $protected;
    static $instance;

     public  function __construct(){
         $this->public    = 'public     <br>';
         $this->private   = 'private    <br>';
         $this->protected = 'protected  <br>';
     }
     protected function tank(){                          //私有方法不能继承，换成public,protected
         if (!isset(self::$instance[get_class()]))
         {
         $c = get_class();
         self::$instance = new $c;
     }
         return self::$instance;
     }

     public function pub_function() {
         echo "you request public function<br>";
         echo $this->public;
     }

     protected  function pro_function(){
         echo "you request protected function<br>";
         echo $this->protected;
     }
     private function pri_function(){
         echo "you request private function<br>";
         echo $this->private;
     }
}


class test1 extends test{
     public function __construct(){
         parent::tank();
         parent::__construct();
     }

     public function tank(){
         echo $this->public;
         echo $this->private;       //Notice: Undefined property: test1::$private
         echo $this->protected;
         $this->pub_function();
         $this->pro_function();
         $this->pri_function();    //Fatal error: Call to private method test::pri_function() from context 'test1'
     }

     public  function pro_extends_function(){
         echo "you request extends_protected function<br>";
     }

     public function pri_extends_function(){
         echo "you request extends_private function<br>";
     }
}
error_reporting(E_ALL);

$test = new test1();
$test -> tank();  //子类和父类有相同名字的属性和方法，实例化子类时，子类的中的属性和方法会盖掉父类的。
?>

public:    test中的public可以被继承。
private:   test中的private不可以被继承。
protected：test中的protected可以被继承。
static：   test中的static可以被继承。