PMPro snippet to allow inherited levels based on a configuration. If a member has any level that matches, they will automatically inherit access to other membership levels as configured.

pmpro-inherit-levels.php

<?php   // do not copy this line.
/**
 * This recipe allows level access to be inherited depending on a member's level.
 *
 * For example, if your membership levels are tiered like Patreon,
 * you may also want to give 'higher' levels access to everything the lower levels have too.
 *
 * 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/
 */

/**
 * The configuration of inherited levels to use.
 *
 * @return array List of inherited level mapping.
 */
function my_pmpro_inherit_levels() {
	// @todo Modify this list based on your needs.
	return [
		[
			// The level ID that will inherit other levels.
			'level_id'            => 3,
			// The list of level IDs that will be inherited.
			'also_give_access_to' => [
				1,
				2,
			],
		],
		[
			// The level ID that will inherit other levels.
			'level_id'            => 4,
			// The list of level IDs that will be inherited.
			'also_give_access_to' => [
				1,
			],
		],
	];
}

/**
 * Add additional inherited levels to the list of levels a user has access to.
 *
 * @param object[] $levels  The list of level objects.
 * @param int      $user_id The user ID.
 *
 * @return object[] The list of level objects.
 */
function my_pmpro_inherit_levels_filter( $levels, $user_id ) {
	$inherit_levels = my_pmpro_inherit_levels();

	$original_level_ids = wp_list_pluck( $levels, 'ID', 'ID' );

	$levels_to_fetch = [];

	// Determine if we have any levels to fetch and add to the $levels list.
	foreach ( $inherit_levels as $inherit_level ) {
		// Skip if the inherit level is not in the current levels list.
		if ( ! isset( $original_level_ids[ $inherit_level['level_id'] ] ) ) {
			continue;
		}

		// Maybe add levels to be fetched.
		foreach ( $inherit_level['also_give_access_to'] as $also_give_access_to ) {
			// If the level is not yet provided, we need to fetch level information.
			if ( ! isset( $original_level_ids[ $also_give_access_to ] ) ) {
				$levels_to_fetch[] = $also_give_access_to;
			}
		}
	}

	// Check if we have additional levels to fetch.
	if ( ! $levels_to_fetch ) {
		return $levels;
	}

	global $wpdb;

	/**
	 * We are going to see if cache is set before doing the query and use that if it is.
	 *
	 * In a default environment with no external object cache, the value is cached in that request and
	 * reduces future MySQL requests. If there is an external object cache like Redis then it will be
	 * persisted until the user level changes.
	 **/
	$cache_key = 'user_' . $user_id . '_additional_levels';

    $additional_levels = wp_cache_get( $cache_key, 'pmpro' );

	if ( false === $additional_levels ) {
		$levels_in = implode( ' OR ', array_fill( 0, count( $levels_to_fetch ), '%d' ) );

		$additional_levels = $wpdb->get_results(
			$wpdb->prepare(
				"
					SELECT
						l.id AS ID,
						l.id as id,
						mu.id as subscription_id,
						l.name,
						l.description,
						l.confirmation,
						l.expiration_number,
						l.expiration_period,
						mu.initial_payment,
						mu.billing_amount,
						mu.cycle_number,
						mu.cycle_period,
						mu.billing_limit,
						mu.trial_amount,
						mu.trial_limit,
						mu.code_id as code_id,
						UNIX_TIMESTAMP(CONVERT_TZ(startdate, '+00:00', @@global.time_zone)) as startdate,
						UNIX_TIMESTAMP(CONVERT_TZ(enddate, '+00:00', @@global.time_zone)) as enddate
					FROM
						{$wpdb->pmpro_membership_levels} AS l
					LEFT JOIN
						{$wpdb->pmpro_memberships_users} AS mu
							ON mu.membership_id = l.id
					WHERE
						l.id IN ( {$levels_in} )
					GROUP BY
						ID
				",
				$levels_to_fetch
			)
		);

		wp_cache_set( $cache_key, $additional_levels, 'pmpro', 3600 );
	}

	// Check if we need to process any additional levels.
	if ( empty( $additional_levels ) ) {
		return $levels;
	}

	// Add the additional levels to the list.
	foreach ( $additional_levels as $additional_level ) {
		$additional_level->initial_payment = pmpro_round_price( $additional_level->initial_payment );
		$additional_level->billing_amount  = pmpro_round_price( $additional_level->billing_amount );
		$additional_level->trial_amount    = pmpro_round_price( $additional_level->trial_amount );

		// Add the additional level to the list.
		$levels[] = $additional_level;
	}

	return $levels;
}

add_filter( 'pmpro_get_membership_levels_for_user', 'my_pmpro_inherit_levels_filter', 10, 2 );