Require a discount code to register for a level

my_pmpro_checkout_checks_require_code_to_register.php

<?php 
/**
 * Require a discount code to register for a level
 *
 * 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_checkout_checks_require_code_to_register( $continue ){
	// only continue if things are okay so far
	if( ! $continue ) {
		return $continue;
	}
		
	global $discount_code;

	// Get the checkout level id.
	$checkout_level = pmpro_getLevelAtCheckout();
	$checkout_level_id = $checkout_level->id;
	
	// ids of levels that require a discount code to check out => the required codes
	$code_levels = array(
		1 => array( 'SOMECODE', 'ANOTHERCODE' ),
		// 4 => array( 'CODE1234', 'CODE5678901' ),
	); 
	
	if( array_key_exists( $checkout_level_id, $code_levels ) && ( empty( $discount_code ) || ! in_array( strtoupper( $discount_code ), $code_levels[$checkout_level_id] ) ) ) {
		pmpro_setMessage( "You must use a valid discount code to register for this level.", "pmpro_error" );
		return false;
	}
	
	return $continue;
}
add_filter( 'pmpro_checkout_checks', 'my_pmpro_checkout_checks_require_code_to_register' );