Check specific WordPress plugins are activated, and alert if not

check-required-plugins.php

<?php
/**
 * Check a list of plugins are activated, and alert if not.
 * @param required_plugins[] should be updated to include the list of required plugins.
 * @param `?show_active_plugins` can be appended to any URL within wp-admin to show a list of currently active plugins.
 */
class checkRequiredPlugin {
    private $required_plugins = [
        'woocommerce/woocommerce.php',
    ];

    private $active_plugins = [];
    private $dependant_plugins = [];

    function __construct() {
        $this->check_required_plugins();
    }

    function active_plugins() {
        if (!function_exists('is_plugin_active')) {
            include_once ABSPATH.'wp-admin/includes/plugin.php';
        }
        return (array)get_option('active_plugins', []);
    }

    function check_required_plugins() {
        $this->active_plugins = $this->active_plugins();
        if (isset($_GET['show_active_plugins'])) {
            echo '<pre style="position:fixed; right:20px; z-index:999; padding:20px; background:white; box-shadow:0 0 60px rgba(0,0,0,.3);">';
            print_r( $this->active_plugins );
            echo '</pre>';
        }
        $this->dependant_plugins = array_intersect($this->active_plugins, $this->required_plugins);
        if (count($this->dependant_plugins) !== count($this->required_plugins)) {
            add_action('admin_notices', [$this, 'required_plugins_notice']);
        }
    }

    function required_plugins_notice() {
        $this->missing_plugins = array_diff($this->required_plugins, $this->dependant_plugins);
        $num_missing = count($this->missing_plugins);
        echo '<div class="notice notice-error">
            <p><strong>There ' . ($num_missing > 1 ? 'are ' : 'is ') . $num_missing . ' missing plugin'.($num_missing > 1 && 's').' for the currently active theme!</strong><br />
                This could seriously affect the front-end of the website and should be fixed urgently!</p>
            <ul>';
            foreach($this->missing_plugins as $missing):
                echo '<li><a href="/wp-admin/plugin-install.php?tab=plugin-information&plugin='.substr($missing, 0, strpos($missing, '/')).'&TB_iframe=true&width=772&height=886" class="thickbox open-plugin-details-modal">' . $missing . '</a></li>';
            endforeach;
            echo '</ul>
         </div>';
    }
}

add_action('admin_init',function(){
    new checkRequiredPlugin;
});