<li><a href="javascript:;" class="downloads" onClick="downloads(this)" id="~link~">~server~</a></li>

<?php
require_once (dirname(__FILE__) . "/include/common.inc.php");
require_once(DEDEINC.'/memberlogin.class.php');
$cfg_ml = new MemberLogin();
if($cfg_ml->IsLogin()){
$login='1';//已经登陆
}
else
{
$login='2';//没有登陆
}
$arr2 = array("login"=>$login);
echo json_encode($arr2);

function downloads(obj)
 {
    $.ajax({
        type: 'POST',
        dataType: 'json',
        url: '/login.php',
        data: {},
        success: function(data) {
            if (data.login == 1)
            {
  
                window.location.href=""+obj.id+"";
                //跳转到下载链接
  
            } else
            {
               alert('请登录后再进行下载');
               //这里可以将你找好的弹出插件放这里
            }
        }
    }); 
}