Require user to have a prerequisite level before purchasing a restricted level. Requires PMPro MMPU to be active.

mmpu_prerequisite_levels.php

<?php

// Copy from below here...

/*
 * Require user to have a prerequisite level before purchasing a restricted level.
 * Requires PMPro MMPU to be active.
 *
 * 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_pmpro_enforce_prerequisite_levels( $okay ) {
	$prerequisite_levels = array( 1, 2 ); // One of these levels is required to buy a restricted level
	$restricted_levels   = array( 3, 4 ); // Levels that require a prerequisite level to be purchased

	global $pmpro_checkout_level_ids;
	
	//only check if things are okay so far and this is a MMPU checkout.	
	if ( $okay && ! empty( $pmpro_checkout_level_ids ) && 0 !== count( array_intersect( $restricted_levels, $pmpro_checkout_level_ids ) ) ) {
		global $pmpro_checkout_level_ids;
		if ( ! pmpro_hasMembershipLevel( $prerequisite_levels ) && 0 === count( array_intersect( $prerequisite_levels, $pmpro_checkout_level_ids ) ) ) {
			pmpro_setMessage("You must purchase a prerequisite level to purchase a restricted level.", "pmpro_error");
			$okay = false;
		}
	}

	return $okay;
}
add_action("pmpro_registration_checks", "my_pmpro_enforce_prerequisite_levels");