Be Careful Where You Keep Your iPhone
Your iPhone is happiest between 62º and 72º Fahrenheit. If it experiences extreme temperatures, like freezing temperatures or heat that hits the 90s, serious problems can occur, especially over time. In those temperatures the battery can be permanently damaged, and other hardware pieces will wear out more quickly.
To make sure your iPhone lasts as long as possible, keep it comfortable. Avoid setting it out in freezing weather, or keeping it next to heaters or in the sun.