renamed plugin php file to a "canonical" (of sorts) name, smae as `wp…

… scaffold plugin <slug>` produces.

wordpress-plugin-woocommerce-purchase-order.php

@@ -0,0 +1,89 @@
+<?php
+/*
+Plugin Name: WooCommerce Checkout Purchase Order Field
+Version: 1.0
+Description: Adds a purchase order field to the checkout.
+Author: Nick Breen
+Author URI: http://foobar.net.nz
+Plugin URI: https://github.com/nickbreen/wordpress-plugin-woocommerce-purchase-order
+*/
+
+// Our new fields.
+$fields = array(
+  'purchase_order_reference' => array(
+    'type' => 'text', //type of field (text, textarea, password, select)
+    'label' => 'Purchase Order Reference', //label for the input field
+    // 'placeholder' => '', //placeholder for the input
+    // 'class' => '', //class for the input
+    // 'required' => false, //true or false, whether or not the field is require
+    // 'clear' => false, //true or false, applies a clear fix to the field/label
+    // 'label_class' => '', //class for the label element
+    // 'options' => '', //for select boxes, array of options (key => value pairs)
+  ),
+);
+
+// We'll put out new fields into the checkout form's $form section
+$form = 'order';
+$form_fields = array(
+  $form => $fields,
+);
+
+// Label for the admin edit order section
+$order_details_label = 'Order Details';
+
+// Validation filters for our new fields.
+$field_filters = array(
+  'purchase_order_reference' => array(
+    'filter' => FILTER_CALLBACK,
+    'flags' => FILTER_REQUIRE_SCALAR,
+    'options' => function ($value) {
+      // if the string does not contain anything unusual
+      return filter_var($value, FILTER_SANITIZE_STRING) === $value ? $value : FALSE;
+    },
+    'notice' => sprintf('<strong>%s</strong> is invalid.', $fields['purchase_order_reference']['label']),
+  ),
+);
+
+add_filter( 'woocommerce_checkout_fields' , function ($fields) use ($form_fields) {
+  return array_merge_recursive($fields, $form_fields);
+});
+
+add_action('woocommerce_checkout_process', function () use ($field_filters) {
+  $filtered_fields = filter_input_array(INPUT_POST, $field_filters, TRUE);
+  foreach ($filtered_fields as $field => $result)
+    // $result === NULL if unspecified, which is OK
+    if (FALSE === $result) // if filter failed, which is not OK
+      wc_add_notice($field_filters[$field]['notice'], 'error');
+});
+
+add_action( 'woocommerce_checkout_update_order_meta', function ($order_id) use ($fields, $field_filters) {
+  $filtered_fields = filter_input_array(INPUT_POST, $field_filters, TRUE);
+  foreach ($filtered_fields as $field => $result)
+    if (FALSE !== $result &&  NULL !== $result)
+      update_post_meta($order_id, $field, $result);
+});
+
+add_filter('woocommerce_email_order_meta_fields', function ($email_fields, $sent_to_admin, $order) use ($fields) {
+  foreach ($fields as $field => $def)
+    $email_fields[] = array(
+      'label' => $def['label'],
+      'value' => get_post_meta($order->id, $field, TRUE),
+    );
+  return $email_fields;
+});
+
+add_action('woocommerce_admin_order_data_after_order_details', function ($order) use ($fields, $order_details_label) {
+  foreach ($fields as $field => $def)
+    if ($value = get_post_meta($order->id, $field, TRUE))
+      $s .= sprintf('<p class="form-field form-field-wide"><label for="%1$s">%2$s:</label><input type="text" id="%1$s" value="%3$s" readonly/></p>', esc_attr($field), esc_html($def['label']), esc_attr($value));
+  if ($s)
+    printf('<div class="clear"></div><h4 style="clear: both">%s</h4><div class="address">%s</div>', $order_details_label, $s);
+});
+
+add_action('woocommerce_thankyou', function ($order_id) use ($fields) {
+  foreach ($fields as $field => $def)
+    if ($value = get_post_meta($order_id, $field, TRUE))
+      $s .= sprintf('<tr><th>%s:</th><td>%s<td/></tr>', esc_html($def['label']), esc_html($value));
+  if ($s)
+    printf('<table class="shop_table order_details">%s</table>', $s);
+},5);