PHP mysql未定义的索引:时间戳

请帮帮我。我面临错误，因为第24行中有未定义的索引：时间戳不确定我缺少了什么。：(感谢您的帮助！提前感谢

$mysql_connect = mysql_connect($db_host, $db_user, $db_pword, $db_name);
$query = "SELECT DATE_FORMAT(`timestamp`,'%d-%m-%Y') from statistics";
if (!$mysql_connect)
die("unable to connect to database: " . mysql_error());
@mysql_select_db($db_name) or die( "Unable to select database");
$result = mysql_query($query);
$response = array();
$stats = array();
while ($row=mysql_fetch_array($result))
{ 
$stats[] = array (
    'date' => $row ['timestamp']
); 
} 
$Response['stats'] = $stats;
$fp = fopen('results.json', 'w');
fwrite($fp, json_encode($Response));
fclose($fp);
die ("Failed to execute Query: " . mysql_error());
mysql_close($mysql_connect);
/*$views = mysql_fetch_array($result);
print("<pre>");
print_r($views);
print("</pre>");*/
?>