WordPress Function: is multisite. Check whether to perform multisite related tasks.

is_multisite-perform-tasks.php

<?php
/**
 * Checks if the plugin is installed and activated 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;

	// Check if we're in network admin and the plugin slug is network activated.
	if ( $network_admin ) {
		if ( is_network_admin() ) {
			if ( is_multisite() && is_plugin_active_for_network( $slug ) ) {
				return true;
			}
		} else {
			return false;
		}
	}

	// Check if the plugin is network-activated on multisite.
	if ( is_multisite() && is_plugin_active_for_network( $slug ) ) {
		return true;
	}
	return false;
}