Add WooCommerce Order number prefix

prefix-wc-order-number.php

<?php // only copy if needed

/**
 * Adds a prefix to the existing WooCommerce order number.
 *
 * @param int $order_id the order ID
 * @param \WC_Order $order the order object
 * @return string the updated order number
 */
function sv_wc_add_order_number_prefix( $order_id, $order ) {
	return "SV-{$order_id}";
}
add_filter( 'woocommerce_order_number', 'sv_wc_add_order_number_prefix' );