Smartphones have largely replaced cameras, newspapers, books, and personal stereos. Next up: Car keys.

Nowadays most cars don't come with actual car keys anyway, just key fobs that let you get into your car and start the engine, often without having to take the fob out of your purse or pocket.

Some carmakers are taking this a step further by creating apps that do the same thing. With the app installed on your phone, you can get into the car and start the engine. It usually works by just being close to the car. But, in some cases, you have to hold the phone near the car door. There's no need to open an app or press anything -- it should work even if the phone is locked.

Hyundai HYMTF Ford's F But such apps haven't gone mainstream yet. Only a handful of carmakers offer the technology and only on certain models. For example, Tesla has offered a version with the Model 3 since 2017.will soon be offering its own app on the 2020 Sonata andLincoln luxury division will offer similar technology on the new Aviator and Corsair SUVs.

Owners of the new Hyundai Sonata will have the option to use their smartphone as a key.

These "phone-as-a-key" apps even allow you to share temporary digital keys with other people. For instance, if I wanted to lend my Sonata to my son for an evening, I could "send" him a temporary key that he could use just for the night. I could also set limits on, not just when, but also where that digital key can be used. Some also have a thin plastic card that I could hand to a valet.

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