在数组递归PHP和MYSQL中对数组进行排序

Question

This is my DB table IMAGE

I Have This code but, the result is this: 

array:2 [▼
  3 => array:1 [▼
    1 => []
  ]
  4 => array:1 [▼
    5 => array:1 [▼
      6 => array:1 [▼
        7 => []
      ]
    ]
  ]
]
But i need organize the code like this:

array:[

0 => 3,
1 => 1,
2 => 4,
3 => 5,
4 => 6,
5 => 7

]

// actually the parameter id is the session user

    public function misSubordinados($id){

    $conn = $this->getEntityManager()->getConnection();
    $sql = 'SELECT s.id_usuario  FROM `sy_supervisores_asesor` AS s
            WHERE s.id_supervisor IN (' . implode(',', [$id]) . ')';
    $stmt = $conn->prepare($sql);
    $stmt->execute();

    $total = array();

    $result = $stmt->fetchAll();

    if(count($result) > 0) {

        foreach ($result as $key => $value) {
                $total[$value['id_usuario']] = $this->misSubordinados($value['id_usuario']);  
        }

    }

    return $total;

    }