我试图选择我的数据库，我设置了这样的日期

$endDate = new DateTime($fetch['loan_end']);
$endDate->add(new DateInterval('P5D'));

此处执行：

$stmtBal = $controller->runQuery("SELECT SUM(amount) 
                                      FROM tbl_paid
                                      WHERE pay_date <= :endDate 
                                      AND (c_id=:c_id AND (transaction_num=:transaction_num))");
    $stmtBal->execute(array(":endDate"         =>  $endDate, 
                            ":c_id"            =>  $get_id,
                            ":transaction_num" =>  $fetch['transaction_num']));

然后xampp给了我这个错误：
可恢复的致命错误：无法将datetime类的对象转换为………中的字符串。。。。。。。。。等