<?php
session_start();
$please_wait = '';
$last = time();
if(isset($_SESSION['last'])) {
$last = $_SESSION['last'];
}
else {
$_SESSION['last'] = $last;
}
//wrap the whole thing in a test for the last hit-time on the page, to avoid abusing NTP servers
if (time() - $last < 1) {
  $wait = time() - $last;
  $please_wait = 'Request limit exceeded, please wait ' . (15 - $wait) . ' s.';
  $server = $vn_response = $mode_response = $stratum_response = $remote_originate = $remote_received
  = $remote_received = $remote_transmitted = $delay_ms = $ntp_time_formatted = $server_time_formatted = $please_wait;
}
else {
  $_SESSION['last'] = time();

  $bit_max = 4294967296;
  $epoch_convert = 2208988800;
  $vn = 3;
  
  $servers = array('localhost');
  $server_count = count($servers);
  
  //see rfc5905, page 20
  //first byte
  //LI (leap indicator), a 2-bit integer. 00 for 'no warning'
  $header = '00';
  //VN (version number), a 3-bit integer.  011 for version 3
  $header .= sprintf('%03d',decbin($vn));
  //Mode (association mode), a 3-bit integer. 011 for 'client'
  $header .= '011';
  
  //echo bindec($header);    
      
  //construct the packet header, byte 1
  $request_packet = chr(bindec($header));
  
  //we'll use a for loop to try additional servers should one fail to respond
  $i = 0;
  for($i; $i < $server_count; $i++) {
    $socket = @fsockopen('udp://'.$servers[$i], 123, $err_no, $err_str,1);
    if ($socket) {
      
      //add nulls to position 11 (the transmit timestamp, later to be returned as originate)
      //10 lots of 32 bits
      for ($j=1; $j<40; $j++) {
        $request_packet .= chr(0x0);
      }
      
      //the time our packet is sent from our server (returns a string in the form 'msec sec')
      $local_sent_explode = explode(' ',microtime());
      $local_sent = $local_sent_explode[1] + $local_sent_explode[0];
      
      //add 70 years to convert unix to ntp epoch
      $originate_seconds = $local_sent_explode[1] + $epoch_convert;
        
      //convert the float given by microtime to a fraction of 32 bits
      $originate_fractional = round($local_sent_explode[0] * $bit_max);
        
      //pad fractional seconds to 32-bit length
      $originate_fractional = sprintf('%010d',$originate_fractional);
        
      //pack to big endian binary string
      $packed_seconds = pack('N', $originate_seconds);
      $packed_fractional = pack("N", $originate_fractional);
  
      //add the packed transmit timestamp
      $request_packet .= $packed_seconds;
      $request_packet .= $packed_fractional;
      
      if (fwrite($socket, $request_packet)) {
        $data = NULL;
        stream_set_timeout($socket, 1);
        
        $response = fread($socket, 48);
        
        //the time the response was received
        $local_received = microtime(true);
        
        //echo 'response was: '.strlen($response).$response;
      }
      fclose($socket);
      
      if (strlen($response) == 48) {
        //the response was of the right length, assume it's valid and break out of the loop
        break;
      }
      else {
        if ($i == $server_count-1) {
          //this was the last server on the list, so give up
          die('unable to establish a connection');
        }
      }
    }
    else {
      if ($i == $server_count-1) {
        //this was the last server on the list, so give up
        die('unable to establish a connection');
      }
    }
  }
  
  //unpack the response to unsiged lonng for calculations
  $unpack0 = unpack("N12", $response);
  //print_r($unpack0);
  
  //present as a decimal number
  $remote_originate_seconds = sprintf('%u', $unpack0[7])-$epoch_convert;
  $remote_received_seconds = sprintf('%u', $unpack0[9])-$epoch_convert;
  $remote_transmitted_seconds = sprintf('%u', $unpack0[11])-$epoch_convert;
  
  $remote_originate_fraction = sprintf('%u', $unpack0[8]) / $bit_max;
  $remote_received_fraction = sprintf('%u', $unpack0[10]) / $bit_max;
  $remote_transmitted_fraction = sprintf('%u', $unpack0[12]) / $bit_max;
  
  $remote_originate = $remote_originate_seconds + $remote_originate_fraction;
  $remote_received = $remote_received_seconds + $remote_received_fraction;
  $remote_transmitted = $remote_transmitted_seconds + $remote_transmitted_fraction;
  
  //unpack to ascii characters for the header response
  $unpack1 = unpack("C12", $response);
  //print_r($unpack1);
  
  //echo 'byte 1: ' . $unpack1[1] . ' | ';
  
  //the header response in binary (base 2)
  $header_response =  base_convert($unpack1[1], 10, 2);
  
  //pad with zeros to 1 byte (8 bits)
  $header_response = sprintf('%08d',$header_response);
  
  //Mode (the last 3 bits of the first byte), converting to decimal for humans;
  $mode_response = bindec(substr($header_response, -3));
  
  //VN
  $vn_response = bindec(substr($header_response, -6, 3));
  
  //the header stratum response in binary (base 2)
  $stratum_response =  base_convert($unpack1[2], 10, 2);
  $stratum_response = bindec($stratum_response);
  //echo 'stratum: ' . bindec($stratum_response);
  
  //calculations assume a symmetrical delay, fixed point would give more accuracy
  $delay = (($local_received - $local_sent) / 2)  - ($remote_transmitted - $remote_received);
  $delay_ms = round($delay * 1000) . ' ms';
  //echo 'delay: ' . $delay * 1000 . 'ms';
  
  $server = $servers[$i];
  
  $ntp_time =  $remote_transmitted - $delay;
  $ntp_time_explode = explode('.',$ntp_time);
  
  $ntp_time_formatted = date('Y-m-d H:i:s', $ntp_time_explode[0]).'.'.$ntp_time_explode[1];
  
  //compare with the current server time
  $server_time =  microtime();
  $server_time_explode = explode(' ', $server_time);
  $server_time_micro = round($server_time_explode[0],4);
  
  $server_time_formatted = date('Y-m-d H:i:s', time()) .'.'. substr($server_time_micro,2);

}
?>

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title></title>
<meta name="description" content="">
<meta name="author" content="">
<style type="text/css">
td{
    width: 160px; height: 20px;
    padding: 4px;
    border: 1px solid #000;
    font-size: 12px;
}
.ntp_response{
    width: 240px;
}
</style>
</head>
<body onload="update_time();">
<table border="0">
<tr>
<td>Server:
<td class="ntp_response"><?php echo $server;?></td>
</tr>
<tr>
<td>VN (version number):</td>
<td class="ntp_response"><?php echo $vn_response;?></td>
</tr>
<tr>
<td>Mode:</td>
<td class="ntp_response"><?php echo $mode_response;?></td>
</tr>
<tr>
<td>Stratum:</td>
<td class="ntp_response"><?php echo $stratum_response;?></td>
</tr>
<tr>
<td>Origin time:</td>
<td class="ntp_response"><?php echo $remote_originate;?></td>
</tr>
<td>Received:</td>
<td class="ntp_response"><?php echo $remote_received;?></td>
</tr>
<td>Transmitted:</td>
<td class="ntp_response"><?php echo $remote_transmitted;?></td>
</tr>
<td>Delay:</td>
<td class="ntp_response"><?php echo $delay_ms;?></td>
</tr>
<td>NTP time:</td>
<td class="ntp_response"><?php echo $ntp_time_formatted;?></td>
</tr>
<td>Server time:</td>
<td class="ntp_response"><?php echo $server_time_formatted;?></td>
</tr>
</table>

<div id="updatetime"></div>
<script type="text/javascript">
var utcSeconds = parseInt(<?php echo $remote_received;?>,10);
var date = new Date(0);
date.setUTCSeconds(utcSeconds);

function update_time() {
  date.setSeconds(date.getSeconds() + 1);
  document.getElementById("updatetime").innerHTML = (date);
  setTimeout("update_time()", 1000);
}

</script>
</body>
</html>