Codeigniter organization chart PHP - php

Question

Whats the best way to output a json like an organization chart? Seems my loop is not good. Can't really get whats the empUnd for every employees. How to make a good loop for this type of chart like get every empUnd over and over until its empty
Create endpoint that shows the organization structure.
/test/chart
Response Format:
{
"empNo: "123",
"name": "Jennifer Sy",
"jobDesc": "CEO",
"empUnd: [
{
"empNo": "456",
"name": "Mike Tolomia",
"jobDesc": "VP",
"empUnd": [{}]
}
]
}
Here's my controller:
public function chart() {
$employeeDetails = $this->api_model->fetch_employees();
foreach ($employeeDetails as $employeeDetail) {
$test = $this->api_model->checkEmployeeUnder($employeeDetail['employeeNumber']);
$result[] = [
'empNo' => $employeeDetail['employeeNumber'],
'name' => $employeeDetail['firstName'],
'empUnd' => $test
];
}
return $this->output->set_content_type('application/json')->set_output(json_encode($result));
}
My Model:
public function fetch_employees()
{
$qry = $this->db->get('employees');
$result = $qry->result_array();
return $result;
}
public function checkEmployeeUnder($reportsTo)
{
$this->db->select('empNo, CONCAT(firstName, '.' , " ", lastName) AS name');
$this->db->from('employees');
$this->db->where('reportsTo', $reportsTo);
$qry = $this->db->get();
$result = $qry->result_array();
return $result;
}