一个Form中2个按钮，PHP后台如何判断提交的是哪一个按钮

<div class="container theme-showcase" role="main">
    <form class="form-horizontal" method="post"  action="<?php echo base_url("manage/audit/user"); ?>" >
        <div class="form-group">
            <label class="col-sm-2 control-label">用户姓名</label>
            <div class="col-sm-10">
                <p class="form-control-static"><?php echo $user['user_name']; ?></p>
            </div>
        </div>
        ................................
        <hr>
        <div class="row">
            <div class="col-xs-6"><button type="submit" name="audit_btn" value="PASS" class="btn btn-success pull-right">审核通过</button></div>
            <div class="col-xs-6"><button type="submit" name="audit_btn" value="NOPASS" class="btn btn-danger pull-left">审核不通过</button></div>
        </div>
        <input type="hidden" name="user_id" value="<?php echo $user['user_id']; ?>" />

    </form>

</div> <!-- /container -->

 /**
     * 用户审核
     */
    public function audit_user()
    {
        $audit_btn = $this->input->post('audit_btn', TRUE);
        if("PASS" == $audit_btn) {
            //审核通过
            $user_audit_status = 'AUDITED';
            //消息内容
            $msg = '用户认证审核已通过，请点击查看认证状态！';
        }
        if("NOPASS" == $audit_btn){
            //审核不通过
            $user_audit_status = 'AUDIT_FAILED';
            $msg = '用户认证审核未通过，请点击查看认证状态！';
        }

        $user_id = $this->input->post('user_id', TRUE);

        ..............................

    }