Get time difference between db time values and today's time in mysql

Question

I want to send email before 2 hours in db time. I developed following code to get all email addresses which has time gap equals to 2 hours or less.

   include('connection.php');
//get current date and time
    $time = date("c");
    $timestamp = strtotime($time);   
    $sendtime = date("Y-m-d H:i:s", $timestamp); 

    echo $sendtime;

 $sql = "SELECT  email FROM confirmed_user WHERE 7200>=TIME_TO_SEC(TIMEDIFF(sendTime, '$sendtime')) diff";
    // $sql = "SELECT sendTime FROM confirmed_user WHERE sendTime='$timestamp'";

$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
    echo "Email: " . $row["email"]. "<br>";
    }
} else {
    echo "0 results";
}
$conn->close();  

So I think my sql query is wrong. I got following error.

Trying to get property of non-object and 'o results'.

I keep date time values as date time format in db( 2016-05-11 12:00:00 ) in sendTime field. Any suggestions?