PHP如何记录页面停留时间的实例

<script>
var dt1 = new Date();
window.onbeforeunload = function(){
  var dt2 = new Date();
  var ms = dt2.getTime() - dt1.getTime();
  var img = new Image();
  img.src = 'log.php?stay_ms=' + ms;
}
</script>

<?php
$refer = '';
if (isset($_SERVER['HTTP_REFERER']))
{
  $refer = $_SERVER['HTTP_REFERER'];
}
$user_agent = '';
if (isset($_SERVER['HTTP_USER_AGENT']))
{
  $user_agent = $_SERVER['HTTP_USER_AGENT'];
}
$ip = $_SERVER['REMOTE_ADDR'];
if (isset($_GET['stay_ms']))
{
  $log = '[' . date("Y-m-d H:i:s") . '] ' . $ip . ' ' . $refer . ' @ ' . number_format($_GET['stay_ms']) . "ms\r\n";
  file_put_contents("log/log_" . date("Y-m-d") . ".txt", $log, FILE_APPEND);
}
if ($_SERVER['QUERY_STRING'] == '' || isset($_GET['day']))
{
  $day = isset($_GET['day']) ? $_GET['day'] : date("Y-m-d");
  $file = "log/log_" .$day . ".txt";
  if (file_exists($file))
  {
    $log = file_get_contents($file);
    echo nl2br($log);
  }
}