php 如何时间观察访问者 停留页面的时间

<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> 

$refer = ''; if (isset($_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);     } }