After form submission want to show success message in the form page

Question

Admin side I submit form to admin-post.php. I want to print success message in the bottom of the form. Iam new in wordpress what i did shown below.

admin.php?page=add-products form code

 <form action="<?php echo admin_url('admin-post.php') ?>" method="post">
                <table>
                <input type="hidden" name="action" value="add_product_from_admin">
                <tr><td>Name</td><td><input type="text" name="pr_name" id="pr_name"></td></tr> 
    <tr><td colspan="2" align="center"><input type="submit" name="pr_submit" id="pr_submit" value="Save Products"></td></tr>
                </table>
            </form>


    add_action( 'admin_post_add_product_from_admin', 'add_product_into_data_base' );



function add_product_into_data_base() {
    //some database operations
wp_redirect(admin_url('admin.php?page=add-products&message=success'));
}

Answer 1

You need to pass the variable and its value in GET method to display the success message which you're already doing.

Now, in your PHP file, display a message based on the GET parameter and its value.

if($_GET['message'] == 'success'){ 
  //display your message 
}

Answer 2

 <form action="<?php echo admin_url('admin-post.php') ?>" method="post">
   <table>
      <input type="hidden" name="action" value="add_product_from_admin">
      <tr>
         <td>Name</td>
         <td><input type="text" name="pr_name" id="pr_name"></td>
      </tr>
      <tr>
         <td colspan="2" align="center"><input type="submit" name="pr_submit" id="pr_submit" value="Save Products"></td>
      </tr>
   </table>
</form>

Handle form request by admin_post with action name - add_product_from_admin

add_action( 'admin_post_add_product_from_admin', 'add_product_into_data_base' );
    
function add_product_into_data_base() {
        //some database operations
        
        // Set a transient with the success message
        set_transient('product_added_success', 'Product added successfully', 5);
    
        wp_redirect(admin_url('admin.php?page=add-products'));
        exit();
}


// Add admin notice for product added successfully
function display_product_added_notice() {
    $message = get_transient('product_added_success');

    if ($message) {
        ?>
        <div class="notice notice-success is-dismissible">
            <p><?php echo esc_html($message); ?></p>
        </div>
        <?php
        // Delete the transient after displaying the notice
        delete_transient('product_added_success');
    }
}
add_action('admin_notices', 'display_product_added_notice');