PHP类中的静态方法

class A {  static function fun()  {  // do somathing  } } 

A::fun()

class MyClass
{
    public $num = 5;

    function __construct()
    {
        $this->num = 10;
    }

    function fun_1()
    {
        echo "I am a public method named fun_1.\n";
        echo "The num of object is {$this->num}.\n";
    }

    static function fun_2()
    {
        echo "I am a static method named fun_2.\n";
    }

    function fun_3($n)
    {
        echo "The arg is {$n}\n";
    }
}

$m = new MyClass;
$m->fun_1();
$m->fun_2();
$m->fun_3('test');

MyClass::fun_1();
MyClass::fun_2();
MyClass::fun_3('test');

lch@localhost:php $ php class_method.php
I am a public method named fun_1.
The num of object is 10.
I am a static method named fun_2.
The arg is test
I am a public method named fun_1.
PHP Fatal error:  Using $this when not in object context in /Users/lch/program/php/class_method.php on line 14