PHP消息：PHP注意：未定义变量：conn in

我已经将PHP版本从5切换到7，在连接到数据库的过程中出现了以下错误

FastCGI在stderr中发送：“PHP消息：PHP通知：未定义变量：conn在/www/admin/Connection.PHP的第14行

从命令行到数据库的连接正常

从标准PHP代码连接到数据库也可以

<?php
class Connection
{
private static $conn;
private $dbHost;
private $dbUser;
private $dbPass;
private $dbName;
public  $dbHandle;
public $languages;

public static function getConnection()
{
        if($conn == null)
                self::$conn = new Connection();
        return self::$conn;
}

protected function __construct()
{
        $this->dbhost = BV_DB_HOST;
        $this->dbuser = BV_DB_USER;
        $this->dbpass = BV_DB_PASS;
        $this->dbname = BV_DB_NAME;
        $this->dbConnect();
}

public function dbConnect()
{
        $this->dbHandle = mysqli_connect(BV_DB_HOST, BV_DB_USER, BV_DB_PASS);
        $tmp = mysqli_select_db($this->dbHandle, BV_DB_NAME);
        mysqli_query($this->dbHandle, "set names utf8");
        return $tmp;
}

public function languages()
{
        return explode(";", BV_LANGS);
}
}
?>

---

如何解决此问题？

我在getConnection（）中的IF语句中更改了条件，如下所示

if(!isset($conn))