如何定义$var id更新数据库在php

这是我的代码，当我按下开始一切工作完美，但当我试图从我的表更新的东西，我得到一个错误通知:未定义的索引:id ..在第4行。如何从表中识别id以便更新行?

if(logged_in() === true){
if(isset($_GET['stop']) ){
$id = $_GET['id'];
$date = mysql_query("SELECT * FROM `orar` WHERE `id`='id'");
$row = mysql_fetch_row($date);
$stop = date('Y/m/d').' '.date('h:i:s');
$update = mysql_query("UPDATE `orar` SET `stop`='$stop'  WHERE `id` = 'id'");
}else{}
if(isset($_POST['start'])){
$user = $user_data['username'];
$start = date('Y/m/d').' '.date('h:i:s');
$add = mysql_query("INSERT INTO `orar` (`id`, `username`, `start`) VALUES ('NULL','$user', '$start')");
echo "<h2>Home</h2><br>
<form method='get' action=''>
<input type='submit' name='stop' class='start_stop_tura-style' value='Stop Tura' />
</form>
";
echo 'Tura start on: '.date('d/m/Y').' at:'.date('h:i:s');
}else{
echo "<h2>Home</h2><br>
<form method='post' action=''>
<input type='submit' name='start' class='start_stop_tura-style' value='Start Tura'/>
</form>";
}
 }else {
 echo '<h2>Home</h2>';
echo 'Login First';
}