Create pseudo-random WooCommerce order numbers

woo-order.php

<?php

// Create pseudo-random order numbers while retaining
// the uniqueness of Woo's order number system
function change_woocommerce_order_number($order_id) {

  // The prefix is prepended to the actual order number
  // It consists of the month number (without leading zeros) followed by a hyphen
  // and then followed by the current minutes (minus leading zeros)
  $prefix = date('n').'-'.ltrim(date('i'), 0); // This is prepended to the order number

  // The suffix is appended to the actual order number
  // It consists of the current seconds (without leading zeros) followed
  // by the current hour (24 hour format)
  $suffix = ltrim(date('s'), 0).date('G');

  // Get rid of the # symbol
  $order_id = str_ireplace("#", "", $order_id);

  // Put it all together...
  $new_order_id = $prefix . $order_id . $suffix;

  return $new_order_id;
}
add_filter('woocommerce_order_number', 'change_woocommerce_order_number');