Blank page/or undefined function mysql_pconnect() in CodeIgniter when upgrading from PHP5 to PHP 7

Run ttr_view1.php when upgrading    from PHP5 to PHP 7, get blank page
<?php
if (!defined('BASEPATH')) die();

class Ttr_view1 extends CI_Controller {

        public function __construct() {
              parent::__construct();
              $this->load->model('auth');
        }

         public function test(){

                $data['styleSheets'] = array('ttr.css', 'shared.css');
                $data['title'] = "Access Denied";

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

        }
}
?>
Add             
 error_reporting(E_ALL);
ini_set('display_errors', '1');
at top, get error message
"Fatal error: Uncaught Error: Call to undefined function mysql_pconnect() in
/var/www/tracs/new6sp/system/database/drivers/mysql/mysql_driver.php:91 Stack trace: #0
 /var/www/tracs/new6sp/system/database/DB_driver.php(115): CI_DB_mysql_driver->db_pconnect() #1
 /var/www/tracs/new6sp/system/database/DB.php(148): CI_DB_driver->initialize() #2
 /var/www/tracs/new6sp/system/core/Loader.php(346): DB(Array, NULL) #3
 /var/www/tracs/new6sp/application/models/stakeholders.php(23): CI_Loader->database() #4"

 In CodeIgniter application/config/database.php
 /var/www/tracs/new6sp/application/config/database.php
 line 56 change
   $db['default']['dbdriver'] = 'mysql';
 to
   $db['default']['dbdriver'] = 'mysqli';