A WP MU plugin that enforces sandbox settings on Paid Memberships Pro. Edit settings as needed in your copy.

pmpro-sandbox.php

<?php
/**
 * Plugin Name:     PM Pro Sandbox
 * Description:     Makes sure that PM Pro is in sandbox mode by forcing the developer's toolkit to run with specific settings, and killing off all pmpro plugins if it isn't found.
 * Author:          Tenseg LLC
 * Author URI:      https://www.tenseg.net
 * Text Domain:     tg-pmpro-sandbox
 * Version:         1.0.0
 *
 * @package         tg-pmpro-sandbox
 */

class TG_PMPro_Sandbox {
	/**
	 * Settings
	 *
	 * @var array of settings
	 */
	private $settings = [
		'expire_memberships' => '1',
		'expiration_warnings' => '1',
		'credit_card_expiring' => '1',
		'ipn_debug' => 'dev@tenseg.net',
		'authnet_silent_post_debug' => '',
		'stripe_webhook_debug' => '',
		'ins_debug' => '',
		'redirect_email' => 'dev@tenseg.net',
		'checkout_debug_email' => 'dev@tenseg.net',
		'checkout_debug_when' => '',
		'view_as_enabled' => false,
		'gateway_environment' => 'sandbox',
	];

	/**
	 * Class constructor
	 */
	public function __construct() {
		add_action( 'plugins_loaded', [$this, 'pmprotoolkit_loader'] );
		add_filter( 'plugin_action_links', [$this, 'disable_plugin_actions'], 10, 4 );
		add_filter( 'default_option_pmprodev_options', [$this, 'force_pmprodev_options'], 10, 2 );
		add_filter( 'option_pmprodev_options', [$this, 'force_pmprodev_options'], 10, 2 );
		add_filter( 'default_option_pmpro_gateway_environment', [$this, 'force_pmpro_gateway_environment'], 10, 2 );
		add_filter( 'option_pmpro_gateway_environment', [$this, 'force_pmpro_gateway_environment'], 10, 2 );
	}

	/**
	 * Checks for the PMPro dev toolkit,manage pmpro plugins accordingly.
	 *
	 * Called by action: plugins_loaded
	 *
	 * @return void
	 */
	public function pmprotoolkit_loader() {
		$active_plugins = get_option( 'active_plugins' );
		// check for the toolkit
		if ( file_exists( WP_PLUGIN_DIR . '/pmpro-toolkit/pmpro-toolkit.php' ) ) {
			require_once WP_PLUGIN_DIR . '/pmpro-toolkit/pmpro-toolkit.php';
			add_action( 'admin_notices', function () {
				if ( 'plugins' === get_current_screen()->id ) {
					echo "<div class='notice'><p>The PM Pro Developer's Toolkit is running, feel free to activate other PM Pro plugins in this sandbox.</p></div>";
				}
			} );
		} else {
			foreach ( $active_plugins as $plugin ) {
				if ( strpos( $plugin, 'pmpro' ) === 0 || strpos( $plugin, 'paid-memberships-pro' ) === 0 ) {
					deactivate_plugins( $plugin );
				}
			}
			add_action( 'admin_notices', function () {
				if ( 'plugins' === get_current_screen()->id ) {
					echo "<div class='error'><p>The PM Pro Developer's Toolkit is required to run PM Pro on this site.</p></div>";
				}
			} );
		}
	}

	/**
	 * Turn off plugin actions for the toolkit.
	 *
	 * @param array $actions
	 * @param string $plugin_file
	 * @param array $plugin_data
	 * @param string $context
	 * @return array
	 */
	public function disable_plugin_actions( $actions, $plugin_file, $plugin_data, $context ) {
		if ( str_starts_with( $plugin_file, 'pmpro-toolkit' ) ) {
			return [];
		}
		return $actions;
	}

	/**
	 * Force the PMPro dev settings based on what is in the config file.
	 *
	 * Called by filter: option_pmprodev_options
	 * Called by filter: default_option_pmprodev_options
	 *
	 * @param mixed $value the option's value
	 * @param string $option the option's name
	 * @return mixed the option's value
	 */
	function force_pmprodev_options( $value, $option ) {
		return $this->settings;
	}

	/**
	 * Force the PMPro gateway environment based on what is in the config file.
	 *
	 * Called by filter: option_pmpro_gateway_environment
	 * Called by filter: default_option_pmpro_gateway_environment
	 *
	 * @param mixed $value the option's value
	 * @param string $option the option's name
	 * @return mixed the option's value
	 */
	function force_pmpro_gateway_environment( $value, $option ) {
		return $this->settings['gateway_environment'];
	}
}

$tg_pmpro_sandbox = new TG_PMPro_Sandbox();