php中使用Session令牌防止Ajax表单重复提交

$_token = md5(microtime()+rand(1,10000));
$_SESSION['_token'] = $_token;

<input type="hidden" value="<?php echo $_token;?>" name="_token"/>

if(!isset($_POST('_token'))){
    echo json_encode(array('status'=>'failed','msg'=>'非法操作！'));
    exit();
}
if(isset($_POST['_token']) && $_POST['_token']!=$_SESSION['_token']){
    echo json_encode(array('status'=>'failed','msg'=>'表单只能提交一次，不能重复提交！'));
    exit();
}