WooCommerce Custom Product Fields

functions.php

<?php 

/**
 * Output ring size field.
 */
function iconic_output_engraving_field() {
	global $product;
  $sizes = $product->get_attribute( 'pa_ring-sizes' ); $sizes = explode(", ",$sizes);
  
  if($sizes[0]!=''){ // if product sizes are available ?>
    <div class="ring-size-field">
      <span class="product-size-label">Choose your ring size:</span>
      <select class="product-size" name="ring-size" id="ring-size">
      <?php foreach($sizes as $size){ ?>
        <option value="<?php echo $size; ?>"><?php echo $size; ?></option>
      <?php } ?>
      </select>
    </div>
  <?php }
}

add_action( 'woocommerce_before_add_to_cart_button', 'iconic_output_engraving_field', 10 );

/**
 * Add selected ring size to cart item.
 *
 * @param array $cart_item_data
 * @param int   $product_id
 * @param int   $variation_id
 *
 * @return array
 */
function iconic_add_engraving_text_to_cart_item( $cart_item_data, $product_id, $variation_id ) {
	$ring_size = filter_input( INPUT_POST, 'ring-size' );

	if ( empty( $ring_size ) ) {
		return $cart_item_data;
	}

	$cart_item_data['ring-size'] = $ring_size;

	return $cart_item_data;
}

add_filter( 'woocommerce_add_cart_item_data', 'iconic_add_engraving_text_to_cart_item', 10, 3 );

/**
 * Display engraving text in the cart.
 *
 * @param array $item_data
 * @param array $cart_item
 *
 * @return array
 */
function iconic_display_engraving_text_cart( $item_data, $cart_item ) {
	if ( empty( $cart_item['ring-size'] ) ) {
		return $item_data;
	}

	$item_data[] = array(
		'key'     => __( 'Ring Size', 'iconic' ),
		'value'   => wc_clean( $cart_item['ring-size'] ),
		'display' => '',
	);

	return $item_data;
}

add_filter( 'woocommerce_get_item_data', 'iconic_display_engraving_text_cart', 10, 2 );

/**
 * Add selected ring size to order.
 *
 * @param WC_Order_Item_Product $item
 * @param string                $cart_item_key
 * @param array                 $values
 * @param WC_Order              $order
 */
function iconic_add_engraving_text_to_order_items( $item, $cart_item_key, $values, $order ) {
	if ( empty( $values['ring-size'] ) ) {
		return;
	}

	$item->add_meta_data( __( 'Ring Size', 'iconic' ), $values['ring-size'] );
}

add_action( 'woocommerce_checkout_create_order_line_item', 'iconic_add_engraving_text_to_order_items', 10, 4 );