mysqli::query() が「mysqli をフェッチできませんでした」データベース接続エラーで失敗するのはなぜですか?

// Connection file
$DBConnect = new mysqli("localhost", "root@localhost", NULL, "Ladle");

// Check for connection error
if ($DBConnect->connect_errno) {
    $ErrorMsgs[] = "The database server is not available. Connect Error is " . $DBConnect->connect_errno . " " . $DBConnect->connect_error . ".";
}

// Class
class EventCalendar {
    private $DBConnect = NULL;

    function __construct() {
        include("inc_LadleDB.php");
        $this->DBConnect = $DBConnect;
    }

    function __destruct() {
        // Close the connection only if it's not closed already
        if (!$this->DBConnect->connect_error) {
            $this->DBConnect->close();
        }
    }

    function __wakeup() {
        // Include the database connection data
        include("inc_LadleDB.php");
        $this->DBConnect = $DBConnect;
    }
    
    // Event adding method
    // ...
}