пользователи должны писать друг другу

Код (Text):

1. <form action="smspost.php" method="POST">
2. <table>
3. <tr><td>First Name:</td>
4. <td><input type="text" name="firstname1" size="40"></td></tr>
5. <tr><td>Message:</td>
6. <td><textarea type="text" name="message" cols="40" rows="4"></textarea></td>
7. <td><input type="submit" name="send" value="Send"></td></tr>
8. </table>
9. </form>
10. </td>

Код (Text):

1. <?php
2.    
3.    include "connect.php";
4.   if(isset($_POST['send'])) {
5. $query = $con->query("SELECT firstname FROM `users` WHERE id ='$id'");
6.    if($query->num_rows>0){
7.     while ($row = $query->fetch_assoc()){
8.        
9.      if($firstname = )
10.      $user_id = $row['id'];
11.    $firstname2 = $row['firstname'];
12.      
13.  
14.   $firstname1  = $_POST["firstname1"];
15.    $senders    = $_POST["$d"];
16.    $message    = $_POST["message"];
17.  
18.         }
19.       }
20.    }
21. $sql = "INSERT INTO `box` (`user_id`,`firstname2`,`firstname1`,`senders`, `message`,`date`)
22.         VALUES ('$user_id','$firstname2','$firstname1','$senders','$message',NOW())";
23.  
24. if (mysqli_query($con, $sql)) {
25.   header("Location:index.php");
26. } else {
27.   echo "Error: " . $sql . "<br>" . mysqli_error($con);
28. }
29.    
30. ?>