Form that prevents duplication and redirects on submission

					<?php

/**
 * Plugin Name:       Contact Form
 * Description:       Display a meta field or a custom field as a block on the front end, supporting ACF fields.
 * Plugin URI:        https://quadlayers.com/create-a-form-in-wordpress/
 * Requires at least: 6.3
 * Requires PHP:      7.4
 * Version:           1.0.0
 * Author:            Eric Thomson
 * License: GNU General Public License v2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 *
 * @copyright Copyright(c) 2024, Eric Thomson
 *
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
    die;
}

/****************************************
 * Contact Form
 */
if ($_SERVER["REQUEST_METHOD"] === "POST") {
    $name    = sanitize_text_field( $_POST["name"] );
    $email   = sanitize_email( $_POST["email"] );
    $message = sanitize_textarea_field( $_POST["message"] );

    // Add code to check if the email already exists in the database
    global $wpdb;
    $table_name   = $wpdb->prefix . 'contact_form_submission';
    $existing_row = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $table_name WHERE email = %s", $email ) );

    if ( $existing_row ) {
        $response = array(
            'success' => false,
            'message' => 'A submission with this email already exists.',
        );
    } else {
        $data = array(
            'name'           => $name,
            'email'          => $email,
            'message'        => $message,
            'submission_time' => current_time( 'mysql' ),
        );
        $insert_result = $wpdb->insert( $table_name, $data );

        if ( $insert_result === false ) {
            $response = array(
                'success' => false,
                'message' => 'Error saving the form data.',
            );
        } else {
            $response = array(
                'success' => true,
                'message' => 'Form data saved successfully.',
            );

            // Add the redirect header for successful submission
            header( 'Location: /ok/' );
            exit;
        }
    }

    // Return the JSON response
    header( 'Content-Type: application/json' );
    echo json_encode( $response );
    exit;
}

/*  Display Form Submissions on Dashboard */
function display_contact_form_submissions_page() {
    global $wpdb;
    $table_name = $wpdb->prefix . 'contact_form_submission';
    $form_data  = $wpdb->get_results( "SELECT * FROM $table_name WHERE name <> '' AND email <> '' AND message <> '' ORDER BY submission_time DESC", ARRAY_A );

    ?>
    <div class="wrap">
        <h1>Contact Form Submissions</h1>
        <table class="wp-list-table widefat fixed striped">
            <thead>
                <tr>
                    <th>Name</th>
                    <th>Email</th>
                    <th>Message</th>
                    <th>Submission Time</th>
                </tr>
            </thead>
            <tbody>
                <?php foreach ( $form_data as $data ) : ?>
                    <tr>
                        <td><?php echo esc_html( $data['name'] ); ?></td>
                        <td><?php echo esc_html( $data['email'] ); ?></td>
                        <td><?php echo esc_html( $data['message'] ); ?></td>
                        <td><?php echo esc_html( $data['submission_time'] ); ?></td>
                    </tr>
                <?php endforeach; ?>
            </tbody>
        </table>
    </div>
<?php }

function register_contact_form_submissions_page() {
    add_menu_page(
        'Contact Form Submissions',
        'Form Submissions',
        'manage_options',
        'contact_form_submissions',
        'display_contact_form_submissions_page',
        'dashicons-feedback'
    );
}
add_action( 'admin_menu', 'register_contact_form_submissions_page' );