function test(){
    static $var = 1; 
    echo $var++.'';
}
test();
test();
test();
//OutPut
//1
//2
//3

class A
{
    private static $a = 1; 
    private $b = 2;
    public function add()
    {
        echo self::$a++.'';
        echo $this->b++.'';
    }
}
$class1 = new A();
$class1->add();
$class1->add();
$class2 = new A();
$class2->add();
$class2->add();
//Output
//1
//2
//2
//3
//3
//2
//4
//3

class A
{
    private static $instance = null;
    private $b = 1;
    public static function get_instance()
    {
        if(self::$instance == null){
            $classname = __CLASS__;
            self::$instance = new $classname(); 
        }
        return self::$instance;
    }
    public function add()
    {
        $this->b++;
    }
    public function show()
    {
        echo $this->b;
    }
}
$a = A::get_instance();
$b = A::get_instance();
//此处$a和$b变量完全相同！
$a->add();
$a->show();
echo '';
$b->show();
//output
//2
//2

class A
{
    private static $instance = null;
    private $b = 1;
    private function __construct()
    {
    //Code in this function
    //could not be get out of the class
    }
    public static function get_instance()
    {
        if(self::$instance == null){
            $classname = __CLASS__;
            self::$instance = new $classname();
        }
        return self::$instance;
    }
    public function add()
    {
        $this->b++;
    }
    public function show()
    {
        echo $this->b;
    }
}
$a = A::get_instance();
$b = A::get_instance();
//此处$a和$b 变量完全相同！
$a->add();
$a->show();
echo '
';
$b->show();
//output
//2
//2
  
//如果尝试用new来实例化的话
$c = new A();
//output
//Fatal error: Call to private A::__construct() from invalid context in
//如果需要A类的实例化对象，只能通过开放的get_instance静态方法进行初始化