将datetime与数据库datetime匹配

I have a exam form. But I want to show that form within given date and time. Means if start date time is 2016-06-24 11:34:04 and end date time is 2016-06-27 00:00:00 If any merson open form between given date and time the form should be open otherwise it will go on another message page. I have a code .But it is not working. Please help.

   <?php
   include("connection.php");

   date_default_timezone_set('Asia/Kolkata');

   $timestamp = time();
   $date_time = date("Y-m-d H:i:s", $timestamp);

   $result=mysql_query("select * from activeform");
                            while($row=mysql_fetch_array($result))
                            {
                                extract($row);

                                if($date_time > '$startdate' and $date_time < '$enddate')
                                {
                                header("Location: myaccount.php");

                                }

                                else{

                                echo"date has been exceeds";
                                }
                                }

    ?>

You can use strtotime if you would like to compare dates in PHP. You may want to look at this answer