php 单例模式

<?php
class DB {
    private static $dh = null;

    private function __construct() {
		self::$dh = mysqli_connect("localhost","root","");
		if (!self::$dh) {
			die('Could not connect: ' . mysql_error());
		}
        echo 'mysql connected...'.PHP_EOL;
    }
    static public function getInstance() {
        if (is_null(self::$dh)) {
            self::$dh = new self();
        }
        return self::$dh;
    }

    public function select() {
        echo 'selecting data...'.PHP_EOL;
    }
}

$db1 = DB::getInstance();
$db2 = DB::getInstance();
var_dump($db1);
var_dump($db1 === $db2);
/*
mysql connected...
object(DB)#1 (0) {
}
bool(true)
*/