Paid Memberships Pro Invoice Code

pmpro-invoice-code-helfer.php

<?php
function pmpro_invoice_code_helfer( $code ) {
	$invoice_code = get_option( 'pmpro_helfer_invoice_code', array() );
	if ( empty( $invoice_code ) ) {
		$invoice_code = array(
			'prefix'    => 'IAW',
			'increment' => 1,
		);
		update_option( 'pmpro_helfer_invoice_code', $invoice_code );
	}
	$increment = absint( $invoice_code['increment'] );
	$increment++;
	$invoice_code['increment'] = $increment;
	update_option( 'pmpro_helfer_invoice_code', $invoice_code );
	return $invoice_code['prefix'] . zeroise( $invoice_code['increment'], 10 );
}

add_filter( 'pmpro_random_code', 'pmpro_invoice_code_helfer' );