Fatal error in form display plugin

The mistake I was doing was defining scope for $wpdb. The correct script looks something like this:
<?php
/*
Plugin Name: Xenon-Result
Plugin URI:  https://developer.wordpress.org/plugins/the-basics/
Description: Basic result display plugin.
Version:     1.0
Author:      Himanshu Gupta
Author URI:  https://developer.wordpress.org/
License:     GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
*/

function installer(){
    include('installer.php');
}
register_activation_hook( __file__, 'installer' ); //executes installer php when installing plugin to create new database

add_action('admin_menu','result_menu'); //wordpress admin menu creation
function result_menu()
{
    add_menu_page('Result','Result','administrator','xenon-result');
    add_submenu_page( 'xenon-result', 'Manage Marks', ' Manage Marks', 'administrator', 'Manage-Xenon-Marks', 'Xenon_Marks' );
}
function Xenon_Marks()
{
    include('new/result-add-marks.php');
}
/*function Result_Form()
{
    include('new/result-form.php');
}*/

function html_form_code() 
{
echo '<form action="" method="post">';
echo '<fieldset>';
echo '<legend>Student Information</legend>';
echo 'Roll Number: <input type="number" min="170001" max="171000" name="rollNumber"><br>';
echo '<input type="submit">';
echo '<input type ="reset">';
echo '</form>';
}

function result_display(){
global $wpdb;
$student_id = $_POST['rollNumber'];
$query = "SELECT * FROM `wp_xenonresult` WHERE `student_id` = $student_id";
$result = $wpdb->get_row($query);
echo $result->student_name;
}

if(isset($_POST['submit']))
{
   result_display();
} 

function display_shortcode() {
    ob_start();
    result_display();
    html_form_code();
    return ob_get_clean();
}

add_shortcode( 'xenon_result_display', 'display_shortcode' );
// Enable shortcodes in text widgets
add_filter('widget_text','do_shortcode');
?>
Related Posts:
