How to hide query strings from url in php?

Im trying to make a message system and when the receiver is reading a message theres a link to reply the message, as follow:

<a href=mail.php?action=compose&toid='".urlencode($viewrow['sender'])."'&subject='RE:+".urlencode($viewrow['subject'])."'&message=".urlencode($viewrow['message']).">Reply</a>

The sender, subject and message are being retrieved from mysql.

The user is seeing something like:

http://www.somesite.com/author/mail.php?action=compose&toid='8'&subject='RE:+test+subject'&message=test+message

What i need to know is if theres a way to hide the subject and the message from reply url, so the user will see on address bar only:

http://www.somesite.com/author/mail.php?action=compose&toid='8'

This is how the subject, message and id is being retrieved from db.

//view message
if (isset($_GET['action']) && $_GET['action'] == inbox) { 
        if(isset($_GET['viewid'])) { 
            $viewid = $_GET['viewid'];
    /*      $viewsql = "select * from mail where reciever='".$userid."' and mail_id=".$viewid; */
            $viewsql = "select * from mail, authors where (mail.sender = authors.id) and (mail.reciever = '".$userid."') and (mail.reciever_deleted ='0') and mail.mail_id = ".$viewid;
            $viewquery = mysql_query($viewsql,$connection) or die(mysql_error());
            $viewrow = mysql_fetch_assoc($viewquery);
            if ($viewrow['reciever'] == $userid) { //check if user is the reciever
            } else { 
                header('Location: mail.php?action=inbox');
                exit; 
            } 
            echo "<h3>Lendo mensagem particular da Caixa de Entrada</h3>";
            echo "<table align=\"center\" width=\"75%\" class=\"sortable\">
                    <tr>
                        <td colspan='2' style=\"text-align:center;font-weight:normal;\">Mensagem particular enviada por ".$viewrow['displayname']." em ".date('d/m/y',strtotime($viewrow['created_at'])).".</td>
                    </tr>
                    <tr>
                        <td colspan='2'>
                            <img style=\"float:left;padding: 5px 15px 5px 2px;width: 65px;\" src=\"".$viewrow['gravatar']."\" alt=\"".$viewrow['displayname']."\" title=\"".$viewrow['displayname']."\" />
                            <div style=\"padding: 8px 5px 2px;\"><span style=\"font-size:1.6em;\">&#8594; </span><b>".$viewrow['subject']."</b></div>
                            <div style=\"padding: 8px 30px 8px 85px;\">".nl2br($viewrow['message'])."<br /></div>
                            <span style=\"float:right;\">
                                <a href=mail.php?action=compose&toid='".urlencode($viewrow['sender'])."'&subject='RE:+".urlencode($viewrow['subject'])."'&message=".urlencode($viewrow['message']).">Responder</a> | <a href=javascript:confirmDelete('mail.php?action=inbox&deleteid=".$viewid."')>Apagar</a>
                            </span>
                        </td>
                    </tr>
                </table>";

            // mark as read by reciever
            $query="update mail set mail_status='read' where reciever = '$userid' and mail_id = '$viewid'"; 
            mysql_query($query,$connection) or die(mysql_error());
        }                   
    }elseif (isset($_GET['action']) && $_GET['action'] == outbox) { ...

I know i need to use post, but how?

I tried but could not make it work.

Sorry for my stupidity..