单例模式小案例

案例一：

//创建一个操作数据库的工具类，要求只链接一次数据库
class DaoMysql{
    //定义需要的属性
    //$mysql_link是数据库链接
    private $mysql_link;
    // $instance是一个静态属性，表示DaoMysql的一个对象实例
    private static $instance = null;
    //构造方法 将构造方法设置为私有
    private function __construct($host,$user,$pwd){
        $this -> mysql_link = @mysql_connect($host,$user,$pwd);
    }
    //写一个静态方法创建对象实例
    public static function getSingleton($host,$user,$pwd){
        //通过 getSingleton来创建对象
        //进行判断如果$instance为空则创建一个对象实例不为空则直接返回$instance
        //instanceof 判断对象实例是否是一个类的对象
        if(!self::$instance instanceof self){
            self:: $instance = new self($host,$user,$pwd); 
        }
        return self:: $instance;
    }
    //阻止克隆
    private function __clone(){}
}
$dao1 = DaoMysql::getSingleton('localhost','root','');
$dao2 = DaoMysql::getSingleton('localhost','root','');
$dao3 = DaoMysql::getSingleton('localhost','root','');
$dao4 = DaoMysql::getSingleton('localhost','root','');
var_dump($dao1,$dao2,$dao3,$dao4);

 案例二：

class Girl{
    private $name;
    private $age;
    private $stature;
    private static $instance = null;
    private function __construct($name,$age,$stature){
        $this -> name = $name;
        $this -> age = $age;
        $this -> stature = $stature;
    }
    public static function getGirl($name,$age,$stature){
        if(!self::$instance instanceof self){
            self::$instance = new self($name,$age,$stature); 
        }
        return self::$instance;
    }
    private function __clone(){}
}
$girl1 = Girl::getGirl('小红',29,160);
$girl2 = Girl::getGirl('小花',23,162);
$girl3 = Girl::getGirl('小玲',26,165);
var_dump($girl1,$girl2,$girl3);