Get much closer to subjects than usual simply by adding a low-cost macro lens attachment to your iPhone's exisiting camera lens -- with no risk of damage.
Step 1: Get a "pocket" magnifier
Mine was sold as a Jeweller's Loupe. It magnifies 8X. I bought it from an Optometrist. You can also get them in model shops, craft centres and hobby outlets.
Step 2: Cover the iPhone lens with the magnifier
Step 3: Move the camera and magnifier together
Here you see the business end of the magnifier and, right in the centre, the iPhone's own little lens. I used a mirror for this shot to prove it really does work.
Step 4: Notes
Make sure the magnifier is centrally positioned over the iPhone's lens, or you will risk cutting off the corners of your pictures. What you see is what you get.
There's no reason why any phone camera should not give similar results.
If you prefer to work single-handed, use Blu-Tak or simillar to fix magnifier temporarily.