how to get last insert id

pls how to get the last insert id after save model?

Code:

$emp_model = new \App\Models\Employee\Employeemodel();
           
            $data = [
                'emp_firstname'     => $fname,
                'emp_lastname'      => $lname,
                'emp_code'          => str_replace("-","",$empcode),
                'emp_birthday'      =>  $dob
            ];
           
           
            if ($emp_model->save($data) === false)
            {
                $this->resp_json['msg'] = array($emp_model->errors());
                return $this->response->setJSON( $this->resp_json);
            }
            $this->resp_json['status'] = 'success';
            $this->resp_json['id'] = $emp_model->id;
            $this->resp_json['msg'] = 'New Employee successed';
            return $this->response->setJSON( $this->resp_json);

i try to write code $emp_model->id is return null;

my model like below

Code:

class EmployeeModel extends Model{
    protected $db;
    protected $table      = 'sk_hr_employee';
    protected $primaryKey = 'id';

    protected $returnType = 'array';
    protected $allowedFields = ['id', 'emp_firstname', 'emp_lastname', 'emp_code', 'emp_birthday'];
    protected $useTimestamps = false;

    protected $validationRules    = [
        'emp_firstname'     => 'required|min_length[3]',
        'emp_lastname'     => 'required|min_length[3]',
        'emp_code'        => 'required|is_unique[sk_hr_employee.emp_code]'
    ];

    protected $validationMessages = [
            'emp_code'   => [
                'required' => 'Sorry. Employee Code Must be fill.',
                'is_unique' => 'Sorry. Employee Code already Taken. Please choose another.'
            ],
            'emp_firstname'     =>[
                'required' => 'Sorry. First Name Must be fill.',
                'min_length' => 'Minimal 3 character'
            ],
            'emp_lastname'     =>[
                'required' => 'Sorry. Last name Name Must be fill.',
                'min_length' => 'Minimal 3 character'
            ]
    ];

    protected $skipValidation     = false;



}

thanks