Nice thought, bad idea.
The better alternative would be putting solar panels on the roofs of cars and then just charging your phone in the car. I think Toyota has a Prius model with panels on the roof that help ventilate the car so it's not as hot sitting in the sun all day.
Anyway, the charge you got from solar panels on such a small area wouldn't be worth the cost involved. Those little solar lights you put in the yard probably use 2 or 3 watts for the actual light. I don't know how many watts an iPhone uses, but it's way more than that.