submit a text on 'Enter' press through textarea with javascript and mysql query

Question

i am new in php and i am trying to make facebook like chat application and trying to insert text through textarea and show it inside chatbox. it is simple to do this with php but in this situation i don't know how to marge javascript with php code.

here is my javascript code which consist on register popup function.

function register_popup(id, name,msg)
{
    for(var iii = 0; iii < popups.length; iii++)
    {   
        //already registered. Bring it to front.
        if(id == popups[iii])
        {
            Array.remove(popups, iii);
            popups.unshift(id);

            calculate_popups();
            return;
        }
    }               

    var element = '<div class="popup-box chat-popup"  id="'+ id +'">';
    element = element + '<div class="msg_head" >';
    element = element + '<div class="popup-head-left" class="close">'+ name +'</div>';
    element = element + '<div class="popup-head-right"><a href="javascript:close_popup(\''+ id +'\');">&#10005;</a></div>';

    element = element + '<div style="clear: both"></div></div><div class="msg_wrap" class="msg_body">'+ msg +'<div class="msg_push"><div class="msg_footer"><textarea class="msg_input" rows="2"></textarea></div></div></div></div>';

    document.getElementsByTagName("body")[0].innerHTML = document.getElementsByTagName("body")[0].innerHTML + element;  

    popups.unshift(id);

    calculate_popups();

}

here is my mysql query for inserting text

public function addMessage($userId, $message) {
    $addResult = false;

    $cUserId = (int) $userId;
    // Escape the message with mysqli real escape
    $cMessage = $this->dbConnection->real_escape_string($message);

    $query = <<<QUERY
      INSERT INTO `chat`(`user_id`, `message`, `sent_on`)
      VALUES ({$cUserId}, '{$cMessage}', UNIX_TIMESTAMP())
QUERY;

    $result = $this->dbConnection->query($query);

    if ($result !== false) {
        // Get the last inserted row id.
        $addResult = $this->dbConnection->insert_id;
    } else {
        echo $this->dbConnection->error;
    }

    return $addResult;
}