Redirect away from the checkout page if a specific parameter is not in the URL

my_pmpro_require_url_param_for_checkout.php

<?php

<?php   // do not copy this line.
/**
 * This recipe will send a user with no level straight to the LEVEL 1 checkout
 * 
 * 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/
 */

/*
 * Redirect away from the checkout page if a specific parameter is not in the URL
 */
function my_pmpro_require_url_param_for_checkout()
{
 	global $pmpro_pages;
	
	if ( pmpro_hasMembershipLevel() ) {
		return;
	}
	
	if( empty( $pmpro_pages ) )
		return;
		
	if ( is_page( $pmpro_pages['checkout'] ) && ( empty( $_REQUEST[ 'level' ] ) || $_REQUEST[ 'level' ] != '1' ) ) {
		wp_redirect( site_url( '/membership-account/membership-checkout/?level=1' ) );
		exit;
	}
}
add_action('template_redirect', 'my_pmpro_require_url_param_for_checkout');