单例模式

$db = IMooc\Database::getInstance();

<?php
namespace IMooc;

class Database
{
    protected $db;

    private function __construct()
    {
    }

    static function getInstance()
    {
        if (self::$db)
        {
            return self::$db;
        }
        else
        {
            self::$db = new self();
            return self::$db;
        }
    }
}