如何在PHPrecursive函数中返回值

Question

我正在尝试构建一个递归树，但是子函数没有返回任何值。我不知道如何在PHP代码中返回值。我在做什么错误请帮忙

 $sql="select * from MenuMaster order by MenuId";
        $result= ExecuteQuery($sql);
    function BuildMenu($result){
        $output ='';
        $output .= '<Ul>';

        while ($row = mysqli_fetch_array ($result)){
            if($row['ParentId']== null || $row['ParentId'] ==0){
                $output .= '<li>'.$row['MenuText'].'</li>';
            }
             $output .= FindChild($output , $row['MenuId']);
        }

        $output.= '</Ul>';
        return $output;
    }
    $finaloutput.= BuildMenu($result);
    echo $finaloutput;




function FindChild($childOutput, $Menuid){

    $sqlChild="select * from MenuMaster where ParentId = $Menuid ";
    $resultChild= ExecuteQuery($sqlChild);
    $Rowcont =mysqli_num_rows($resultChild);


    if($Rowcont > 0){
      $childOutput.='<Ul>';

        while ($row1 = mysqli_fetch_array ($resultChild)){
            $childOutput.= '<li>'.$row1['MenuText'].'</li>';
            FindChild($childOutput,$row1['MenuId']);
        }
        $childOutput.='</Ul>';
    }
        return  $childOutput;
      }