/**
 * Add extra checkout fields
 */

add_filter('woocommerce_checkout_fields', 'cc_add_extra_checkout_fields');

function cc_add_extra_checkout_fields($fields) {

  $fields['extra_fields'] = array(

    'student_name' => array(
    'type'         => 'text',
    'required'     => true,
    'label'        => __('Student first and last name'),
    'placeholder'  => '',
    'class'        => array('student-name')
    ),
    'student_year' => array(
    'type'         => 'text',
    'required'     => true,
    'label'        => __('Year'),
    'placeholder'  => '',
    'class'        => array('student-year')
    ),
  );

  return $fields;

}

// display the extra field on the checkout form

add_action('woocommerce_checkout_after_customer_details' ,'cc_extra_checkout_fields');

function cc_extra_checkout_fields(){ 

  $checkout = WC()->checkout(); ?>

<?php 

  // because of this foreach, everything added to the array in the previous function will display
  foreach ( $checkout->checkout_fields['extra_fields'] as $key => $field ) : ?>

  <?php woocommerce_form_field( $key, $field, $checkout->get_value( $key ) ); ?>

  <?php endforeach; ?>

<?php }


/**
 * Update the value given in custom fields
 */

add_action('woocommerce_checkout_update_order_meta', 'custom_checkout_field_update_order_meta');

function custom_checkout_field_update_order_meta($order_id) {

  if (!empty($_POST['student_name'])) {
    update_post_meta($order_id, 'student_name',sanitize_text_field($_POST['student_name']));
  }

  if (!empty($_POST['student_year'])) {
    update_post_meta($order_id, 'student_year',sanitize_text_field($_POST['student_year']));
  }

}


/**
 * Display field values on the order edit page
 */

add_action('woocommerce_admin_order_data_after_billing_address', 'cc_custom_checkout_field_display_admin_order_meta', 10, 1);
 
function cc_custom_checkout_field_display_admin_order_meta($order) {
  echo '<p><strong>'.__('Student name').':</strong> ' . get_post_meta( $order->get_id(), 'student_name', true ) . '</p>';
  echo '<p><strong>'.__('Student year').':</strong> ' . get_post_meta( $order->get_id(), 'student_year', true ) . '</p>';
}


/**
 * Add a custom field (in an order) to the emails
 */

add_filter('woocommerce_email_order_meta_fields', 'cc_email_order_meta_fields', 10, 3);

function cc_email_order_meta_fields($fields, $sent_to_admin, $order) {
  $fields['student_name'] = array(
    'label' => __('Student name'),
    'value' => get_post_meta($order->get_id(), 'student_name', true),
    );
    $fields['student_year'] = array(
    'label' => __('Student year'),
    'value' => get_post_meta($order->get_id(), 'student_year', true),
  );

  return $fields;

}