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 }