Check if a plugin is active for a site

check-plugin-active-site.php

<?php
/**
 * Checks to see if an asset is activated or not.
 *
 * @since 1.0.0
 *
 * @param string $path Path to the asset.
 * @param string $type Type to check if it is activated or not.
 *
 * @return bool true if activated, false if not.
 */
public static function is_activated( $path, $type = 'plugin' ) {

	// Gets all active plugins on the current site.
	$active_plugins = self::is_multisite() ? get_site_option( 'active_sitewide_plugins' ) : get_option( 'active_plugins', array() );
	if ( in_array( $path, $active_plugins, true ) ) {
		return true;
	}
	return false;
}

// Needs helper function below.
/**
 * Checks if the plugin is on a multisite install.
 *
 * @since 1.0.0
 *
 * @param bool   $network_admin Check if in network admin.
 * @param string $slug Plugin slug.
 *
 * @return true if multisite, false if not.
 */
public static function is_multisite( $network_admin = false, $slug = 'alerts-dlx' ) {
	if ( ! function_exists( 'is_plugin_active_for_network' ) ) {
		require_once ABSPATH . '/wp-admin/includes/plugin.php';
	}
	$is_network_admin = false;
	if ( $network_admin ) {
		if ( is_network_admin() ) {
			if ( is_multisite() && is_plugin_active_for_network( $slug ) ) {
				return true;
			}
		} else {
			return false;
		}
	}
	if ( is_multisite() && is_plugin_active_for_network( $slug ) ) {
		return true;
	}
	return false;
}