mypmpro-only-allow-one-level-between-groups.php

<?php //do not copy
/**
 * This recipe ensures that the member can only hold one level across all groups. 
 * Signing up for more than one group across any level groups will cancel all other levels.
 *
 * 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/
 *
 */

/**
 * Cancel memberships from other groups when a user checks out for a new one.
 * Keeps memberships in the same group.
 */
function my_pmpro_cancel_other_group_memberships( $user_id, $morder ) {

    // Get all memberships the user currently holds (after checkout this may include both).
    $user_levels = pmpro_getMembershipLevelsForUser( $user_id );

    if ( empty( $user_levels ) ) {
        return;
    }

	$new_level = pmpro_get_group_id_for_level( $morder->membership_id );

    foreach ( $user_levels as $level ) {
        // Skip the new level (we don’t want to cancel it).
        if ( $level->id == $morder->membership_id ) {
            continue;
        }

		$current_level = pmpro_get_group_id_for_level( $level->id );

		if( $current_level == $new_level ) {
			continue; // Same group, skip cancellation.
		}

		// Cancel the membership.
		pmpro_cancelMembershipLevel( $level->id, $user_id );
    }
}
add_action( 'pmpro_after_checkout', 'my_pmpro_cancel_other_group_memberships', 20, 2 );