AImager-Singleton

单个进程/线程中避免生成多个实例，在使用框架的时候常常用到

php实现

class Singleton {
    private static $_instance = null;

    // defend new instance external
    protected function __construct() {

    }

    public static function getInstance () {
        if(self::$_instance === null) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }
}