Add a custom text field to a WooCommerce Product

kia-sample-custom-field-for-woocommerce.php

<?php
/**
 * Plugin Name: Sample Customizable Product Field for WooCommerce
 * Plugin URI: https://www.kathyisawesome.com/add-a-custom-field-to-woocommerce-product/
 * Description: Add a custom text field to a WooCommerce Product
 * Version: 1.1.0
 * Author: Kathy Darling
 * Author URI: http://kathyisawesome.com
 * Requires at least: 5.2.0
 * Tested up to: 5.2.2
 * WC requires at least: 3.6.0   
 * WC tested up to: 3.7.0   
 *
 * Text Domain: kia-plugin-textdomain
 * Domain Path: /languages/
 *
 * Copyright: © 2022 Kathy Darling.
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 *
 */

/**
 * Display input on single product page
 */
function kia_custom_option() {
    $value = isset( $_POST['_custom_option'] ) ? sanitize_text_field( $_POST['_custom_option'] ) : '';
    printf( '<p><label>%s<input name="_custom_option" value="%s" /></label></p>', __( 'Enter your custom text', 'kia-plugin-textdomain' ), esc_attr( $value ) );
}
add_action( 'woocommerce_before_add_to_cart_button', 'kia_custom_option', 9 );

/**
 * Validate when adding to cart
 * 
 * @param bool $passed
 * @param int $product_id
 * @param int $quantity
 * @return bool
 */
function kia_add_to_cart_validation( $passed, $product_id, $qty ) {

    if ( isset( $_POST['_custom_option'] ) && sanitize_text_field( $_POST['_custom_option'] ) == '' ) {
        $product = wc_get_product( $product_id );
        wc_add_notice( sprintf( __( '%s cannot be added to the cart until you enter some custom text.', 'kia-plugin-textdomain' ), $product->get_title() ), 'error' );
        return false;
    }

    return $passed;

}
add_filter( 'woocommerce_add_to_cart_validation', 'kia_add_to_cart_validation', 10, 3 );

/**
 * Add custom data to the cart item
 * 
 * @param array $cart_item
 * @param int $product_id
 * @return array
 */
function kia_add_cart_item_data( $cart_item, $product_id ) {

    if ( isset( $_POST['_custom_option'] ) ) {
        $cart_item['custom_option'] = sanitize_text_field( $_POST['_custom_option'] );
    }

    return $cart_item;

}
add_filter( 'woocommerce_add_cart_item_data', 'kia_add_cart_item_data', 10, 2 );

/**
 * Load cart data from session
 * 
 * @param array $cart_item
 * @param array $other_data
 * @return array
 */
function kia_get_cart_item_from_session( $cart_item, $values ) {

    if ( isset( $values['custom_option'] ) ) {
        $cart_item['custom_option'] = $values['custom_option'];
    }

    return $cart_item;

}
add_filter( 'woocommerce_get_cart_item_from_session', 'kia_get_cart_item_from_session', 20, 2 );

/**
 * Get item data to display in cart
 * 
 * @param array $other_data
 * @param array $cart_item
 * @return array
 */
function kia_get_item_data( $other_data, $cart_item ) {

    if ( isset( $cart_item['custom_option'] ) ) {

        $other_data[] = array(
            'key' => __( 'Your custom text', 'kia-plugin-textdomain' ),
            'display' => sanitize_text_field( $cart_item['custom_option'] )
        );

    }

    return $other_data;

}
add_filter( 'woocommerce_get_item_data', 'kia_get_item_data', 10, 2 );

/**
 * Add meta to order item
 * 
 * @param int $item_id
 * @param string $cart_item_key
 * @param array $item
 */
function kia_add_order_item_meta( $item, $cart_item_key, $values ) {

    if ( ! empty( $values['custom_option'] ) ) {
		$item->add_meta_data( 'custom_option', wc_clean( $values['custom_option'] ), true );           
    }

}
add_action( 'woocommerce_checkout_create_order_line_item', 'kia_add_order_item_meta', 10, 3 );


/**
 * Restore custom field to product meta when product retrieved for order item.
 * Meta fields will be automatically displayed if not prefixed with _
 * 
 * @param WC_Product $product 
 * @param WC_Order_Item_Product $order_item
 * @return array
 */
function kia_order_item_product( $product, $order_item ) {

    if ( $order_item->get_meta( 'custom_option' ) ) {
        $product->add_meta_data( 'custom_option', $order_item->get_meta( 'custom_option' ), true );
    }

    return $product;

}
add_filter( 'woocommerce_order_item_product', 'kia_order_item_product', 10, 2 );

/**
 * Customize the display of the meta key in order tables.
 * 
 * @param string $display_key
 * @param obj[] $meta
 * @param WC_Order_Item_Product $order_item
 * @return string
 */
function kia_order_item_display_meta_key( $display_key, $meta, $order_item ) {

    if ( $meta->key == 'custom_option' ) {
        $display_key =  __( 'Your custom text', 'kia-plugin-textdomain' );
    }

    return $display_key;

}
add_filter( 'woocommerce_order_item_display_meta_key', 'kia_order_item_display_meta_key', 10, 3 );

/**
 * Order Again
 * 
 * @param array $cart_item
 * @param WC_Order_Item_Product $order_item
 * @param WC_Order $order
 * @return array
 */
function kia_order_again_cart_item_data( $cart_item, $order_item, $order ) {

    if ( $order_item->get_meta( 'custom_option' ) ) {
        $cart_item['custom_option'] = $order_item->get_meta( 'custom_option' );
    }

    return $cart_item;

}
add_filter( 'woocommerce_order_again_cart_item_data', 'kia_order_again_cart_item_data', 10, 3 );

[13-Apr-2022 19:05:58 UTC] woocommerce_add_order_item_meta is deprecated since version 3.0.0! Use woocommerce_new_order_item instead.

It seems WooCommerce has deprecated the woocommerce_add_order_item_meta now. Any updates for compatibility with WooCommerce 3.0+?

Thank you for sharing this code!

@mitchellknight The fix was noted in https://www.kathyisawesome.com/add-a-custom-field-to-woocommerce-product/ but I have updated it here too. Needs to switch to the woocommerce_checkout_create_order_line_item hook.