Receipt Numeration

<?php
add_action( 'wpo_wcpdf_after_order_data', 'wpo_wcpdf_receipt_number', 10, 2 );
function wpo_wcpdf_receipt_number ($template_type, $order) {
	global $wpo_wcpdf;
	if ($template_type == 'receipt') {
		// get number from order
		$receipt_number = get_post_meta( $order->id, 'Receipt Number', true );
		// get number from database if no number present yet
		if (empty($receipt_number)) {
			$receipt_number = get_option( 'wcpdf_next_receipt_number', 1001 ); // change 1001 to define the first number
			// store number in order
			update_post_meta($order->id, 'Receipt Number', $receipt_number);
			// increase Receipt number in database
			update_option( 'wcpdf_next_receipt_number', $receipt_number+1 );
		}
		?>
		<tr class="receipt-number">
			<th>Receipt Number:</th>
			<td><?php echo $receipt_number; ?></td>
		</tr>
		<?php
	}
}
?>