WooCommerce Custom Fields with Custom Nonce and Validation

woocommerce_nonce.php

<?php
/**
 * Create custom checkout field and nonce.
 *
 * @param WC_Checkout $checkout The checkout object.
 *
 * @return void
 */
function custom_checkout_field( $checkout ) {

	woocommerce_form_field(
		'custom_checkout_field',
		array(
			'type'     => 'text',
			'class'    => array( 'custom-checkout-text-field form-row-wide' ),
			'label'    => __( 'Custom Field', 'plugin-text-domain' ),
			'required' => true,
		),
		$checkout->get_value( 'custom_checkout_field' )
	);

	$allowed_html = array(
		'input' => array(
			'type'  => array(),
			'class' => array(),
			'name'  => array(),
			'id'    => array(),
			'value' => array(),
		),
	);

	/**
	 * wp_kses()
	 * @link https://developer.wordpress.org/reference/functions/wp_kses/
	 */
	echo wp_kses(
		'<input type="hidden" class="input-hidden" name="_customfieldnonce" id="_customfieldnonce" value="' .
		/**
		 * wp_create_nonce()
		 * @link https://developer.wordpress.org/reference/functions/wp_create_nonce/
		 */
		wp_create_nonce( 'custom_text_field_action' ) .
		'" />',
		$allowed_html
	);
}

/**
 * Validate the nonce & custom field being required before checkout.
 *
 * @return void
 */
public function custom_field_validation() {

	if ( ! empty( $_REQUEST['_customfieldnonce'] ) &&
		/**
		 * wp_verify_nonce()
		 * (int|false) 1 if the nonce is valid and generated between 0-12 hours ago, 2 if the nonce is valid and generated between 12-24 hours ago. False if the nonce is invalid.
		 * @link https://developer.wordpress.org/reference/functions/wp_verify_nonce/#return
		 */
		wp_verify_nonce( sanitize_text_field( stripslashes_from_strings_only( $_REQUEST['_customfieldnonce'] ) ), 'custom_text_field_action' ) == 1 ) {
		// Check if set, if its not set add an error.
		if ( empty( wc_get_post_data_by_key( 'custom_checkout_field' ) ) ) {
			wc_add_notice( __( 'Please enter a value.', 'plugin-text-domain' ), 'error' );
		}
	} else {
		// Bad or expired nonce.
		wc_add_notice( __( 'Expired or invalid submission!.', 'plugin-text-domain' ), 'error' );
	}

}

/**
 * Update the order meta with the custom text field value.
 *
 * @param  integer $order_id The ID of the order you want meta data for.
 *
 * @return void
 */
public function update_order_meta( $order_id ) {

	if ( ! empty( $_REQUEST['_customfieldnonce'] ) &&
		/**
		 * wp_verify_nonce()
		 * (int|false) 1 if the nonce is valid and generated between 0-12 hours ago, 2 if the nonce is valid and generated between 12-24 hours ago. False if the nonce is invalid.
		 * @link https://developer.wordpress.org/reference/functions/wp_verify_nonce/#return
		 */
		wp_verify_nonce( sanitize_text_field( stripslashes_from_strings_only( $_REQUEST['_customfieldnonce'] ) ), 'custom_text_field_action' ) == 1 ) {
		// Check if set, if its not set add an error.
		if ( empty( wc_get_post_data_by_key( 'custom_checkout_field' ) ) ) {
			update_post_meta( $order_id, '_custom_checkout_field', wc_get_post_data_by_key( 'custom_checkout_field' ) );
		}
	} else {
		// Bad or expired nonce.
		wc_add_notice( __( 'Expired or invalid submission!', 'plugin-text-domain' ), 'error' );
	}

}