self与static的区别

1.self 与 static 关键字用法区别：

class A {
    public static function model()
    {
        self::getModel();//第一种方式
        static::getModel();//第二种方式
    }

    public static function getModel()
    {
        echo 'I am a!' . PHP_EOL;
    }
}
A::model();

输出：
I am a!
I am a!

class B extends A {
    public static function getModel()
    {
        echo 'I am b!' . PHP_EOL;
    }
}
A::model();
B::model();

输出：
I am a!
I am a!
I am a!
I am b!

2.static方法里面不能用this调用本类里面的属性和方法

class A {
    private $property = 'test';
    public static function showMessage()
    {
        echo $this->property;
    }
}
A::showMessage();

执行这个方法PHP会报一个错误：Fatal error: Using $this when not in object context in C:\PHPnow-1.5.6\htdocs\test.php on line 32

3.static在类中定义的静态变量在内存中只保存一份

class User {
    private static $count = 0;

    public function __construct()
    {
        ++self::$count;
    }
    public function getCount()
    {
        echo self::$count . PHP_EOL;
    }
    public function __destruct()
    {
        --self::$count;
    }
}

$user1 = new User;
$user1->getCount();
$user2 = new User;
$user2->getCount();
unset($user1);
$user2->getCount();

输出结果：
1
2
1