FreeScout is the super lightweight and powerful free open source help desk and shared inbox built with PHP. It can be easily deployed even on a shared hosting.
Here are just some of its features: slick modern design, free iOS and Anroid apps, seamless email integration, auto replies, internal notes, collision detection, modules allowing to integrate with WhatsApp, Facebook or Telegram, Knowledge Base, Live Chat, CRM, Customer Portal, Canned Responses and much more.