function ajax(url, parameters) {
    http_request = false;
    
    if( window.XMLHttpRequest ){ // Mozilla, Safari, ...
        http_request = new XMLHttpRequest();
        if( http_request.overrideMimeType ) http_request.overrideMimeType('text/html');
    } else if( window.ActiveXObject ){ // IE
        try {
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e){
            try {
                http_request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e){}
        }
    }
    if( !http_request ){
        alert('ERROR: cannot execute ajax...');
        return false;
    }
    
    http_request.open('POST', url, false);
    http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    http_request.setRequestHeader("Content-length", parameters.length);
    http_request.setRequestHeader("Connection", "close");
    http_request.send(parameters);
    
    if( http_request.readyState == 4 && http_request.status == 200 ){
        return http_request.responseText;
    } else return false;
}