disable/hide button using condition in controller

how to disable.hide one submit button (Register) in the controller

My query should be like this

 $query = $this->db->query("select count(*) as cnt from student where brcode=$brdata and sem=$semester and inst_code=$inst_code");
        $row = $query-> row_array ();
        $max_id = $row ['cnt']; 
        $max_id1 = (int) ($max_id);
        $cntt = $max_id1;
      

       
        if ( $cntt > 0 )
        {
          Disable button Register how?
        }
        else
        {
enable button Register }

If you are not already using views, you should consider it, but here's a simple example how to pass data to view and then in view display different HTML, depending on values.

Controller:

PHP Code:

public function index()
{
    // ...

    $showRegistrationButton= false;

    // data to pass to view
    $data = [
        'showRegistrationButton' => $showRegistrationButton,
        'pageTitle' => 'My page title'
    ];

    $this->load->view('myview', $data);
} 


View file:

PHP Code:

<h1><?= $pageTitle ?></h1>
<?php if ($showRegistrationButton) : ?>
    <a href="<?= site_url('registration-controller') ?>">Register</a>
<?php else: ?>
    <div>Already registered</div>
<?php endif ?>