chat Mesagem visualizada chat php

[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>

Editado Abril 12, 2016 por Rede Otaku