<script type="text/javascript">
    function PopUpShow(i) {
        $("#popup" + i).show();
    }

    function PopUpHide(i) {
        $("#popup" + i).hide();
    }

    function sendComment(message,user) {
        if (message == ""){
            document.getElementById("resultComment").innerHTML = "";
            return;
        } else {
            if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest();
            } else {
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            xmlhttp.onreadystatechange = function() {
                if (this.readyState == 4 && this.status == 200) {
                    document.getElementById("resultComment").innerHTML = this.responseText;
                }
            };
            xmlhttp.open("GET", "https://yoursite.com/comments.php?comment=" + message + "&user=" + user, true);
            xmlhttp.send(message);
        }
    }

    function PopUpSend(i, message, user) {
        $("#popup" + i).hide();
        sendComment(message);
    }
</script>

if (!empty($_SESSION['user'])){
                $user = $_SESSION['user'];
                $login = (string)$user->Login;
                echo '<a href="javascript:PopUpShow('.$i.')">Добавить комментарий</a>
                    <div class="b-popup" id="popup'.$i.'">
                        <div class="b-popup-content">
                            <form method="post" class="form-horizontal">
                                <label>Добавить комментарий</label>
                                <br />
                                <textarea type="text" name="comment" rows="8" cols="77"></textarea>
                                <a class="btn btn-default" onclick="PopUpHide('.$i.');" role="button" style="width: 100%">Закрыть</a>
                                <a class="btn btn-default" onclick="PopUpSend('.$i.',this.value,'.$login.');" role="button" style="width: 100%">Отправить</a>
                            </form>
                        </div>
                    </div>';
            }

<a class="btn btn-default" onclick="PopUpSend(0,'123',admin);" role="button" style="width: 100%">Отправить</a>