Display Success Message After Form Submission in WordPress

Code

<?php
// Hook into the form submission process
add_action('init', 'wp_dudecom_handle_form_submission');

function wp_dudecom_handle_form_submission() {
    // Check if the form is submitted
    if (isset($_POST['wp_dudecom_form_submit'])) {
        
        // Verify the nonce for security
        if (!isset($_POST['wp_dudecom_form_nonce']) || !wp_verify_nonce($_POST['wp_dudecom_form_nonce'], 'wp_dudecom_form_action')) {
            return;
        }

        // Process form data here
        // Example: $name = sanitize_text_field($_POST['name']);

        // Set a transient to display the success message
        set_transient('wp_dudecom_form_success', true, 30);
        
        // Redirect to avoid resubmission
        wp_redirect(add_query_arg('form_submitted', 'true', wp_get_referer()));
        exit;
    }
}

// Display the success message
add_action('wp_footer', 'wp_dudecom_display_success_message');

function wp_dudecom_display_success_message() {
    if (get_transient('wp_dudecom_form_success')) {
        echo '<div class="wp-dudecom-success-message">Thank you! Your form has been successfully submitted.</div>';
        
        // Delete the transient after displaying the message
        delete_transient('wp_dudecom_form_success');
    }
}

// Example form with nonce field
function wp_dudecom_example_form() {
    ?>
    <form method="post" action="">
        <?php wp_nonce_field('wp_dudecom_form_action', 'wp_dudecom_form_nonce'); ?>
        <input type="text" name="name" required>
        <input type="submit" name="wp_dudecom_form_submit" value="Submit">
    </form>
    <?php
}

// Shortcode to display the form
add_shortcode('wp_dudecom_form', 'wp_dudecom_example_form');
?>