获取其他用户的ID及其在数据库中的信息，

Question

I am having a problem on how to get the id of the selected user along with it's information while I am logged in as an another user, using mysql, the two tables is innerjoined with each other I just dont know how to get the ID of other user, and passing it on other database

if(isset($_POST['send'])) {

$query =  "SELECT * FROM tblmessage INNER JOIN tblusers ON tblmessage.userid=tblusers.userid";
$res = mysqli_query($con,$query);

while ($row = mysqli_fetch_assoc($res)) 
{
    $receiverID = $row['userid'];
}

    $sender = $_SESSION['Email'];
    $receiver = $_POST['email'];
    $message = $_POST['textmessages'];
    $date = date("Y-m-d h:i:sa");

    $q = 'INSERT INTO `tblmessage`(`id`,`sender_name`,`receiver_name`,`receiver_id`,`message_text`,`date_time`,`userid`)
       VALUES("","'.$sender.'","'.$receiver.'","'.$receiverID.'","'.$message.'","'.$date.'","'.$user_id.'")';
    $r = mysqli_query($con, $q);

    if($r)
    {
            echo"<script>alert('Message Sent')</script>
                <script>window.location.href='messages.php?id=$receiverID';</script>";
    }   
    else
    {
        echo $q;
    }

}