ntp_check.php

<?php

// getTimeFromNTP source: https://gist.github.com/bohwaz/6d01bf00fdb4721a601c4b9fc1007d81

/**
 * Returns UNIX timestamp from a NTP server (RFC 5905)
 *
 * @param  string  $host    Server host (default is pool.ntp.org)
 * @param  integer $timeout Timeout  in seconds (default is 10 seconds)
 * @return integer Number of seconds since January 1st 1970
 */
function getTimeFromNTP($host = 'pool.ntp.org', $timeout = 10)
{
    $socket = stream_socket_client('udp://' . $host . ':123', $errno, $errstr, (int)$timeout);
    $msg = "\010" . str_repeat("\0", 47);
    fwrite($socket, $msg);
    $response = fread($socket, 48);
    fclose($socket);
    // unpack to unsigned long
    $data = unpack('N12', $response);
    // 9 =  Receive Timestamp (rec): Time at the server when the request arrived
    // from the client, in NTP timestamp format.
    $timestamp = sprintf('%u', $data[9]);
    // NTP = number of seconds since January 1st, 1900
    // Unix time = seconds since January 1st, 1970
    // remove 70 years in seconds to get unix timestamp from NTP time
    $timestamp -= 2208988800;
    return $timestamp;
}

if(abs(time() - getTimeFromNTP()) > 10) trigger_error('Server time appears to be incorrect.', E_USER_ERROR);