解决AJAX在不同浏览器中兼容性问题(能兼容到IE5.5版本)

<script type="text/javascript">
    /*    支持火狐浏览器
    var ajax = new XMLHttpRequest();
    ajax.open('get', 'ajax.php', true);//同步刷新
    
    ajax.onreadystatechange = function(){
        if (ajax.readyState == 4)
        {
            alert(ajax.responseText);
        }
    }
    */
    var $ajax;
    
    try{
        $ajax = new XMLHttpRequest();
    }
    catch (e)
    {
        try
        {
            $ajax = new ActiveXObject('Microsoft.XMLHTTP');            
        }
        catch (e)
        {
            try
            {
                $ajax = new ActiveXObject('Msxm12.XMLHTTP');
            }
            catch (e)
            {
            
            }
        }
    }

    $ajax.open('get', 'ajax.php', true);

    $ajax.onreadystatechange = function() {
        
        if ($ajax.readyState == 4)
        {
            alert($ajax.responseText);
        }
    }
    $ajax.send(null);
  </script>