Bootstrap modal - dont close after submitting form - php

Question

I have a Bootstrap template, anyhow I am creating a modal [popup] on my page where the user can submit a form. Although The form works fine but on my PHP code I orignally have that after submitting, it'll show a message either if it worked it would say something like 'Your message has been posted' or if it was an error it would show the error.
But unfortunately once I click 'Post' it posts, but when I click 'Open modal' button again to show the modal popup it shows the message then saying it was submitted, meaning I cant post again without reloading the page.
How would I be able to make it show the message without closing the modal, and also when I close, then open it back up [without reloading] it'll return back to my form instead of having a message.
Thanks.
My code: [Might not be exact as I cut a few un-needed things out]
<?php
echo '
<div style="float:left;margin-top:-51px;margin-left:141px;"> <!-- Button trigger modal -->
<button class="btn btn-primary btn-sml" data-toggle="modal" data-target="#myModal">
Send private message
</button>
<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
<h4 class="modal-title" id="myModalLabel">Send a message to '.$row["username"].'</h4>
</div>
<div class="modal-body"> ';
if($_POST[add2]){
$subject = strip_tags($_POST['title']);
$content = $_POST['content'];
$date = date("F jS, Y g:i a");
$receiver = $row["id"];
$sender = $member["id"];
if(!$content){
echo 'All fields are required';
}else{
$sql = $dbh->prepare("INSERT INTO `inbox` (`receiver`, `sender`, `subject`, `content`, `date`, `status`) VALUES ('$receiver', '$sender', '$subject', '$content', '$date', 'unread')");
$sql->execute();
$q = $sql->fetch(PDO::FETCH_ASSOC);
if($sql){
echo '<center>Message sent!</center>';
}else{
echo '<strong>Error:</strong> '.mysql_error();
}
}
}else{
echo '
<div style="overflow:auto;"><form method="post">
<div style="float:left;width:550px;">Subject<br>
<input type="text" name="title" id="title" class="form-control" value="" size="50" maxlength="40">
<br>Message<br>
<textarea style="height:85px;max-width: 550px;" class="form-control" name="content" placeholder="Enter your message here..." ></textarea></div>
<br><hr></div>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
<input type="submit" style="margin-left:10px;" class="btn btn-primary" name="add2" value="Post">
</form>
</div> '; }
?>