Your smartphone is already a very capable camera for photography and video. The Ulanzi Smartphone Video Rig lets you take it to the next level by attaching microphones and lights. It has 2 standard shoe mounts for your accessories. You also get handles on both sides for more stable video and better grip.
You can mount this to a tripod, slider, or dolly. It comes with a standard smartphone clamp too. Costs under $50 at this time.