Keeps Stripe subscription attached to membership level after purchasing an Addon Package.

my_pmproap_keep_membership_subscription_linked.php

<?php

/**
 * Keeps Stripe subscription attached to membership level after purchasing an Addon Package.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmproap_keep_membership_subscription_linked( $user_id, $morder ) {
	if ( empty( $_REQUEST['ap'] ) ) {
		// User isn't purchasing an addon package, so we don't need to keep a subscription linked.
		return;
	}

	// Get all previous memberships for the user, including inactive.
	$prev_levels = pmpro_getMembershipLevelsForUser( $user_id, true ); // True to also get inactive levels.
	if ( count( $prev_levels ) === 1 ) {
		// This is the user's first purchase. We don't need to keep a subscription linked.
		return;
	}

	$new_level  = end( $prev_levels );
	$prev_level = prev( $prev_levels );

	// Check if the user purchased the same level that they previously had.
	if ( $new_level->id !== $prev_level->id ) {
		// They purchased a different level, so we don't need to keep a subscription linked.
		return;
	}

	// Check if the new level that they purchased is already recurring.
	if ( pmpro_isLevelRecurring( $new_level ) ) {
		// The new level is already recurring, so we don't need to keep a subscription linked.
		return;
	}

	// Make sure that the old level that they purchased was recurring.
	if ( ! pmpro_isLevelRecurring( $prev_level ) ) {
		// The old level isn't recurring, so we don't need to keep a subscription linked.
		return;
	}

	// Move subscription data from the old level to the new level.
	global $wpdb;
	$wpdb->get_results( $wpdb->prepare( "UPDATE $wpdb->pmpro_memberships_users SET billing_amount = %d, cycle_number = %d, cycle_period = %s WHERE user_id = %d AND status = 'active'", $prev_level->billing_amount, $prev_level->cycle_number, $prev_level->cycle_period, $user_id ) );
}
add_action( 'pmpro_after_checkout', 'my_pmproap_keep_membership_subscription_linked', 10, 2 );