auto incremental registration number not working

following youtube tots


i want to auto increase registration number

model code

Code:

public function auto() {
        $registerNO = "SPPS";
        $query = 'select max(registerNO) as registerNO_auto from student';
        $data = $this->db->query($query)->row_array();
        $max_registerNO = $data['registerNO_auto'];
        $max_registerNO2 = (int)substr($max_registerNO,7,3);
        $registerNOcount = $max_registerNO2+1;
        $registerNO_auto = $registerNO."-".sprintf('%03s', $registerNOcount+1);
        return $registerNO_auto;
    }

view code

Code:

<?php
                             $registerNO = $this->student_m->auto();
                            echo form_input([
                                'name'=> 'registerNO',
                                'class'=>'form-control',
                                'value'=>set_value('registerNO', $registerNO),
                                'readonly'=>'true'
                            ]);
                            ?>

 my output is static as "SPPS-0001" there is no increament whenever i try adding new member