<?php
function recursive_member_pool_find_d($id) /// $id is the reffer id.
    {
    //// here table name is Member, and A meand leg_a, B means leg_b, C means leg_c and D means leg_d and serial is autoincrement value of the database.
        $result = Member::select('id', 'A', 'B', 'C', 'D')->where('id', $id)
                        ->where(function ($query) {
                            $query->where('A', '0')
                                  ->orWhere('B', '0')
                                  ->orWhere('C', '0')
                                  ->orWhere('D', '0');
                        })
                        ->orderBy('serial', 'ASC')
                        ->first();
        if (empty($result)) {
            $result = Member::select('id', 'A', 'B', 'C', 'D')->where('position', $id)
                            ->where(function ($query) {
                                $query->where('A', '0')
                                      ->orWhere('B', '0')
                                      ->orWhere('C', '0')
                                      ->orWhere('D', '0');
                            })
                            ->orderBy('serial', 'ASC')
                            ->first();
        }
        if ($result->A == 0) {
            return [
                'position' => $result->id,
                'leg'      => 'A',
            ];
        } else if ($result->B === 0) {
            return [
                'position' => $result->id,
                'leg'      => 'B',
            ];
        } else if ($result->C === 0) {
            return [
                'position' => $result->id,
                'leg'      => 'C',
            ];
        } else if ($result->D === 0) {
            return [
                'position' => $result->id,
                'leg'      => 'D',
            ];
        } else {
            $data = Member::select('id')->where('position', $id)->orderBy('serial', 'ASC')->get();
            foreach ($data as $result):
                if ($result->id !== $id):
                    return recursive_member_pool_find_d($result->id); /// calling the same function
                endif;
            endforeach;
        }
    }