PDO Update using Forms

I am trying to incorporate a button onto a page that will when clicked will run a update statement and execute it, but it does not seem to work. Any ideas on how to fix it?

<?php
$query = $handler->query('SELECT * FROM listings WHERE Accept=0;');
while($r = $query->fetch()) {
    echo '<td>', $r['CompID'], '</td>','<td>', $r['Name'], '</td>','<td>',$r['Email'], '</td>','<td>', $r['Phone'], '</td>',
    '<td>','<a href="' . $r['Name'] . '.php">Company page</a>', '</td>';
    echo '</tr>';
    }
echo '</table>';
if(isset($_POST['app'])){
    $app =$_POST['app'];
}

if($app = !null){
    $sql = "UPDATE listings SET Accept=1 WHERE CompID='.$app.';";
    $Appquery = $handler->prepare($sql);
    $Appquery->bindParam(':.$app.', $_POST['app'], PDO::PARAM_INT);
    $Appquery->execute();
    }

?>
<br>
<p>Plese enter the Company ID of the company to approve</p>
<br>
<form action="Admin3.php" method ="POST">
<input type="text" name="app"/>
<input type="submit" value="Approve"/>
</form>