Rede Otaku

Desenvolvi um chat utilizando php e gostaria de saber como posso colocar um aviso igual o do facebook de "mensagem visualizada"? Esse é o código do meu chat <?php $hidden_class = ''; if (!isset($sk['chat']['recipient']['id'])) { $hidden_class = ' hidden'; } ?> <div id="chat_<?php echo $sk['chat']['recipient']['id'] ?>" class="chat-wrapper<?php echo $hidden_class; ?>"> <div class="chat-header" onclick="javascript:$('.chat-content-toggler').toggle();"> <div class="float-left"> <?php $active_class = ''; if ($sk['chat']['recipient']['online'] == true) { $active_class = ' active'; } ?> <i class="icon-circle online-icon<?php echo $active_class; ?>"></i> <span class="chat-recipient-name"> <?php echo substr($sk['chat']['recipient']['name'], 0, 15); ?> </span> </div> <div class="float-right"> <i class="icon-remove close-icon" onclick="SK_closeChat();" title="Close chat"></i> </div> <div class="float-clear"></div> </div> <div class="chat-content-toggler"> <div class="chat-messages"> <?php if (isset($sk['chat']['recipient']['id'])) { $array_data = array('recipient_id' => $sk['chat']['recipient']['id']); $messages = SK_getMessages($array_data); if (is_array($messages)) { foreach ($messages as $sk['message']) { if ($sk['message']['account']['id'] == $sk['user']['id']) { echo SK_getPage('chat/outgoing-text'); } else { echo SK_getPage('chat/incoming-text'); } } } } ?> </div> <?php $chat_textarea = false; if ($sk['chat']['recipient']['message_privacy'] == "following" && SK_isFollowing($sk['user']['id'], $sk['chat']['recipient']['id'])) { $chat_textarea = true; } elseif ($sk['chat']['recipient']['message_privacy'] == "everyone") { $chat_textarea = true; } if ($chat_textarea == true) { ?> <div class="chat-textarea"> <textarea class="auto-grow-input" onfocus="SK_focusChat();" onkeyup="SK_sendChatMessage(this.value,<?php echo $sk['chat']['recipient']['id']; ?>,event);"></textarea> <div class="advanced-options"> <i class="icon-smile cursor-hand" onclick="javascript:$('.chat-emoticons-wrapper').toggle();"></i> <div class="chat-emoticons-wrapper"> <?php $emoticons = SK_getEmoticons(); if (is_array($emoticons)) { foreach ($emoticons as $emo_code => $emo_icon) { echo '<img src="' . $emo_icon . '" width="16px" onclick="addEmoToChat(\'' . $emo_code . '\');">'; } } ?> </div> </div> </div> <?php } ?> </div> </div>