Spend too much time doing pretty much anything on your phone, and you'll find that it's starting to get uncomfortably warm. That's a consequence of using too much too often, and whether or not it's a problem is a topic for something else entirely. Aside from potentially doing damage to your phone itself, that heat can also do unspeakable things to your battery life, too. It doesn't matter if you're talking on it or if you were just careless enough to leave it baking on the hood of your car (you heathen) — heat is heat and it's going to hurt your battery.
Smartphones use lithium-ion (or Li-ion) batteries, and for the sake of everything we're talking about, you should know a couple basic things. One is that what makes them special is that the flow of molecules goes in two different directions, and that allows for recharging. The second is that they slowly degrade over time, and that's probably made you say, "Duh." Listen up, we're not done.
Every time you recharge your battery, you're generating a huge amount of heat. That's a big deal, because every time the internal temperature of your phone climbs 10 degrees Celsius, the speed of your battery's chemical reaction doubles. That means it's shortening your battery life and the amount of charge it can hold, starting your battery on its ever-so-slow death and decay. It ends up being something of a vicious cycle: battery life decreases, you charge it more, which generates more heat, and your battery life gets even shorter. Say you're keeping your phone at an average internal temperature of 104 degrees Fahrenheit, and an average of 40 percent charged. You're going to lose about 15 percent of your battery's capacity in 12 months, and if you rely on it all the time, that's a massive decline.
So, what can you do? Keep your phone's vents clean, and keep it away from external heat sources. If you live in Arizona, that just means you're going to have to do what any sane person would do, and move somewhere bearable. For your phone's sake.