Ciao prova così:


<script language="php" src="http://www.nomesito.com/counter.php"></script>


e ti consiglio di modificare il tuo script così:


<?php
$file = "count.txt";
$fd = fopen($file,"r+");
$count = fgets($fd, 4096);
fclose($fd);
$count = $count++;
$fd = fopen($file, "w+");
$string = fwrite($fd, $count);
fclose($fd);
?>