Add apartment number field in checkout billing adress.

woocommerce-checkout-apartmentno-field.php

<?php
/**
 * Plugin Name: WooCommerce Checkout Apartment No. Field 
 * Plugin URI: https://github.com/hdytsgt
 * Description: Add apartment number field in checkout billing adress.
 * Version: 1.0.0
 * Author: Hidayat Sagita
 * Author URI: https://github.com/hdytsgt
 * Developer: Hidayat Sagita
 * Developer URI: https://github.com/hdytsgt
 * Text Domain: woocommerce-checkout-apartmentno-field
 * Domain Path: /i18n/languages/
 *
 * Copyright: 2018 Hidayat Sagita.
 * 
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */

/**
 * Quit if accessed directly
 */
if( !defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Make sure WooCommerce is active
 */
if( !in_array( 'woocommerce/woocommerce.php', apply_filters( 'active_plugins', get_option( 'active_plugins' ) ) ) ) {
	return;
}

/**
 * Display apartment field in checkout billing address
 * 
 * @param  Array $fields
 * @return Array         
 */
function wc_apartment_show_field( $fields ) {

	// Declare the new field
	$fields[ 'billing' ][ 'billing_apartment_no' ] = [
		'type' => 'text',
		'placeholder' => _x( 'Apartment number (optional)', 'placeholder', 'woocommerce' ),
		'required'    => false
	];

	// Reorder fields so it makes sense 
	$fields_order = [
		'billing_first_name', 'billing_last_name', 'billing_company', 'billing_email', 
		'billing_phone', 'billing_country', 'billing_address_1', 'billing_address_2',
		'billing_apartment_no', 'billing_city', 'billing_postcode', 'billing_state'
	];

	foreach( $fields_order as $field ) {
		$ordered_fields[ 'billing' ][ $field ] = $fields[ 'billing' ][ $field ];
	}

	// Merge with default fields
	$fixed_fields = array_merge( $fields, $ordered_fields );

    return $fixed_fields;
}

/**
 * New field validation.
 * Make sure only allow number.
 * 
 * @param  Array $data  
 * @param  Object $errors
 * @return Void         
 */
function wc_apartment_validate_field( $data, $errors ) {

	if( $data[ 'billing_apartment_no' ] && !is_numeric( $data[ 'billing_apartment_no' ] ) ) {
		$errors->add( 'billing_apartment_no',  _x( 'Please make sure <strong>Apartment number</strong> is only number', 'placeholder', 'woocommerce' ), 'error' );
	}
}

/**
 * Make sure new field is registered for visibility in any location : WP Admin, Account and Thank You page
 * 
 * @param  Array  $fields
 * @param  Object $order
 * @return Array        
 */
function wc_order_formatted_billing_address( $fields, $order ) {
	
	$fields[ 'billing_apartment_no' ] = get_post_meta( $order->get_id(), '_billing_apartment_no', true );
	return $fields;
}

/**
 * Replace new field tag with real value
 * 
 * @param  Array $tags
 * @param  Array $address      
 * @return Array               
 */
function wc_formatted_address_replacements( $tags, $address ) {
	
	$tags[ '{billing_apartment_no}' ] = isset( $address[ 'billing_apartment_no' ] ) ? $address[ 'billing_apartment_no' ] : '';
	return $tags;
}

/**
 * Make sure localized address format to include new field
 * 
 * @param  Array $formats
 * @return Array          
 */
function wc_localisation_address_formats( $formats ) {

	$formats[ 'default' ] = "{name}\n{company}\n{address_1}\n{address_2}\n{billing_apartment_no}\n{city}\n{state}\n{postcode}\n{country}";
	return $formats;
}

/**
 * Initialize this plugin after ensuring WooCommerce is loaded. 
 * 
 * @return Void
 */
function wc_checkout_apartmentno_field_init() {

	// Register field in checkout fields hook
	add_action( 'woocommerce_checkout_fields', 'wc_apartment_show_field' );

	// Validate new field
	add_action( 'woocommerce_after_checkout_validation', 'wc_apartment_validate_field', 10, 2 );

	// Make sure the new field visible in any location : Backend, Tracking and Thank You page
	add_filter( 'woocommerce_order_formatted_billing_address', 'wc_order_formatted_billing_address', 10, 2 );
	add_filter( 'woocommerce_formatted_address_replacements', 'wc_formatted_address_replacements', 10, 2 );
	add_filter( 'woocommerce_localisation_address_formats', 'wc_localisation_address_formats', 10, 2 );

}

add_action( 'plugins_loaded', 'wc_checkout_apartmentno_field_init', 11 );