Require an Addon Package to be purchased for certain membership levels. [Paid Memberships Pro]

require-ap-for-checkout.php

<?php
/**
 * This code recipe requires an Addon Package to be purchased for a particular level. Please adjust the level ID on line 18.
 * 
 * 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_require_ap_for_checkout( $continue ) {
    global $pmpro_level;
	
	// If things aren't okay, just bail.
    if ( ! $continue ) {
        return $continue;
    }

    if ( ! isset( $_REQUEST['ap'] ) && intval( $pmpro_level->id ) == 1 ) {
        global $pmpro_msg, $pmpro_msgt;
	$pmpro_msg = __( "Warning: This level requires an Addon Package to be purchased.", 'paid-memberships-pro' );
	$pmpro_msgt = "pmpro_error";
        $continue = false;
    }

    return $continue;
}
add_filter( 'pmpro_registration_checks', 'my_pmpro_require_ap_for_checkout', 10, 1 );