Число нажатий на кнопку

<?php

$c = file('counter.txt');

if (isset($_GET['add']))
{
$c[0]++;
$fp = fopen('counter.txt','w');
fputs($fp,$c[0]);
fclose($fp); 
}

echo $c[0];

?>

var req;

function processReqChange() {
if (req.readyState == 4){ 
if (req.status == 200){
document.getElementById('counter').innerHTML=req.r esponseText; 
}; 
};
};

function loadXMLDoc(url) { 
if (window.XMLHttpRequest) { 
req = new XMLHttpRequest(); 
req.onreadystatechange = processReqChange; 
req.open("GET", url, true); 
req.send(null); 
} else if (window.ActiveXObject) { 
req = new ActiveXObject("Microsoft.XMLHTTP"); 
if (req) { 
req.onreadystatechange = processReqChange;
req.open("GET", url, true); 
req.send(); 
}; 
};
};

function counter(v){
loadXMLDoc('counter.php'+v); 
}; 

<script language="javascript" type="text/javascript" src="/counter.js"></script>

<script language="javascript"> 
if (window.print) { document.write(''); }
</script><form><input type="button" name="print" value="Распечатать" onclick="javascript:window.print();counter('?add');"></form> 

<p>Распечатали <span id="counter">0</span> человек</p>

<p><img src="./kupon_files/kupon_print.jpg" alt="Распечатай купон на скидку"></p>