PHP设计模式之：单例模式

<?php

//单例模式
class Database{
    private $_db;
    public static $_instance;

    //private的构造函数，防止被实例化
    private function __construct(){
        $this->__db = pg_connect('dbname=test');
    }

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

    //private的clone方法，防止对象clone后产生多个实例对象
    private function __clone(){
        trigger_error('Clone is not allow' ,E_USER_ERROR);
    }

    public function query($sql){
        return pg_query($this->$_db,$sql);
    }
}

//调用方法
$conn = Database::getInstance();
$conn->query(' select * from user ');