AJAX probleem

    var xmlhttp = null;
    
    if(window.XMLHttpRequest){
        xmlhttp = new XMLHttpRequest();
    }else{
        try{
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }catch(e){
            window.alert("Uw browser ondersteunt het XMLHttpRequest object niet!");
        }
    }
    
    function getTimestamp(){
        xmlhttp.open("GET", "time.php", true);
        xmlhttp.onreadystatechange = showTimestamp;
        xmlhttp.send('');
    }
    
    function showTimestamp(){
        if(xmlhttp.readyState == 4){
            window.alert(xmlhttp.responseText);
        }
    }

setInterval("getTimestamp()", 5000);

<?php
$datum = date("j F Y");
$dagvanweek = date("l");
$arraydag = array(
"Zondag",
"Maandag",
"Dinsdag",
"Woensdag",
"Donderdag",
"Vrijdag",
"Zaterdag"
);
$dagvanweek = $arraydag[date("w")];
$arraymaand = array(
"Januari",
"Februari",
"Maart",
"April",
"Mei",
"Juni",
"Juli",
"Augustus",
"September",
"Oktober",
"November",
"December"
);
$datum = date("j ") . $arraymaand 
[date("n") - 1] . date(" Y"); 
echo $dagvanweek.' '.$datum.' '.date('H:i:s', time());   


?>