echo "<div class=messagess>";
    $msg=$db->query("SELECT * FROM `messages` WHERE `recipient`='{$_SESSION['id']}'");
    while($r=mysqli_fetch_array($msg)){
        $users=$db->query("SELECT * FROM `users` WHERE `id`='{$r['author']}'");
        while($r2=mysqli_fetch_array($users)){
            $avatar=select('background', 'avatar', 'id', $r['author'], true);
            if($r2['id']==2){?>
                <script>
                    $(document).ready(function() {
                        $('.ava').addClass('mm');
                    });
                
                </script>
            <?php }
            echo "<a href=/messages?type=private&id=".$r2['id'].">";
            echo "<div class=user>";
            echo "<img class=ava src=".photo($avatar[0])."><br>";
            echo "<b><h2 class=nick>".$r2['nickname']."</h2></b>";
            echo '<span class=ms>'.$r['message'].'</span>';
            echo "</div>";
            echo '</a>';`.mm{
        border: 4px solid green;
    } 
   
  
  <?php
...
$style = '';
if( $r2['id'] == 2 ) {
    $style = 'mm';
}
echo "<img class='$style' src=".photo($avatar[0])."><br>";
...