If we 404, and the slug matches a pmpro discount code, redirect

my_pmpro_pre_handle_404.php

/*
	If we 404, and the slug matches a discount code, redirect
  
  Add this code to a custom plugin
*/
function my_pmpro_pre_handle_404($preempt, $wp_query) {
	global $wpdb;

	//bail if PMPro is not installed
	if(empty($wpdb->pmpro_discount_codes_levels))
		return $preempt;

	//look for a code
	$possible_code = sanitize_text_field(str_replace('/', '', strtolower($_SERVER['REQUEST_URI'])));
	$code = $wpdb->get_row("SELECT * FROM $wpdb->pmpro_discount_codes WHERE code = '" . esc_sql($possible_code) . "' LIMIT 1");

	//redirect to checkout for the first level found
	if(!empty($code)) {
		$code_level = $wpdb->get_row("SELECT * FROM $wpdb->pmpro_discount_codes_levels WHERE code_id = '" . $code->id . "' ORDER BY initial_payment DESC");

		//found one, redirect
		if($code_level) {
			wp_redirect(pmpro_url('checkout', '?level=' . $code_level->level_id . '&discount_code=' . $code->code));
			exit;
		}
	}

	return $preempt;	
}
add_filter('pre_handle_404', 'my_pmpro_pre_handle_404', 10, 2);