<script type="text/javascript">
    window.onload = function() {
        var timer = parseInt(document.getElementById("timer").innerHTML);
        setInterval(function() {
            timer++;
            document.getElementById("timer").innerHTML = timer;
        }, 1000);
    }
</script>
<?php
$utente = "pippo";
$file = $utente . ".timer.txt";
if (!file_exists($file)) {
    $fr = fopen($file, 'w');
    fwrite($fr, time());
    fclose($fr);
}
$fr = fopen($file, 'r');
while (!feof($fr)) {
    $timer = fgets($fr);
}
fclose($fr);
echo "Il timer è iniziato da <span id='timer'>" . (time() - $timer) . "</span> secondi";