--- JavaScript ---

function getXMLHttp() {
    var xmlHttp;
    try {
        xmlHttp = new XMLHttpRequest();
    } catch (e) {
        try {
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                return false;
            }
        }
    }
    return xmlHttp;
}

function count() {
    var xmlHttp = getXMLHttp();
    xmlHttp.open("POST", "count.php", true);
    xmlHttp.send(null);
}


--- PHP (count.php) ---

<?php

$pdo = new PDO('mysql:host=MYHOST;dbname=MYDATABSE;', 'USERNAME', 'PASSWORD');
$sql = "UPDATE table SET field = field + 1";
$pdo->exec($sql);

?>