I just had the same experience.
The experience was like the power button got crazy. My phone was calling the emergency number, open the camera, restart,... everything by itself.
It happens after an afternoon at the beach.
The solution was for me
1. Playing with the button, pushing the edges
2. Remove the case, so the phone can cool down properly
3. Try to keep the phone turn off (it was turning on by itself)
4. Charge the phone
5. Download the latest update (not sure it was related)
After a while, it worked
But I really thought the phone was broken, and hijacked.