XmlHttpRequest对象封装的函数，支持ie和firefox

<script>

function createXMLHttp()
{
    var ret = null;
    try {
        ret = new ActiveXObject('Msxml2.XMLHTTP');
    }
    catch (e) {
        try {
            ret = new ActiveXObject('Microsoft.XMLHTTP');
        }
        catch (ee) {
            ret = null;
        }
    }
    if (!ret && typeof XMLHttpRequest != 'undefined')
        ret = new XMLHttpRequest();

    return ret;
}

</script>