php 递归返回值的问题

请大佬帮我看看递归循环查找上级在循环里面打印数据全部拿到了，但是return 返回数据后却是空，请问是怎么回事？？

    public function for_enter($data=array(),$level=0,$shu=array(),$s=0){
        $ui = M('ui');
        $array = array();
        $where['is_enter'] = 1;
        foreach ($data as $k => $v) {
            $child = $ui->where(array('code'=>$v['pcode']))->select();
            if (empty($child)) {
                return '';
            }else{
                if ($child[0]['level'] == 4) {
                    $this->for_enter($child,$level,'');
                }else{
                    if ($child[0]['is_enter'] == 1) {
                    if (($level - $child[0]['level']) == 3) {
                        $shu['three'] = $child[0];
                        $this->for_enter($child,$level,$shu);
                    }
                    
                    if (($level - $child[0]['level']) == 2) {
                        $shu['tow'] = $child[0];
                        $this->for_enter($child,$level,$shu);
                    }
                    if (($level - $child[0]['level']) == 1) {
                        //判断是几级
                        $shu['one'] = $child[0];
                        $this->for_enter($child,$level,$shu);
                    }
                    }
                }
            }
            $array[] = $shu;
        }
        
        return $array;
    }

打印数据