There are several myths floating around Internet discussion forms related to recharging iPhones. The myths take on several different forms, but can be simplified as:
1: By using a more power iPad charger you can recharge your iPhone faster.
2: By using a more power iPad charger you will damage your battery or ruin your iPhone.
By using a simple in-line power meter we can measure that when a more powerful iPad charger is used, the iPhone does not draw any additional current from the charger than when a less powerful iPhone charger is used.
Since no additional current is entering the iPhone when a more powerful charger is used, it is not charging any faster. The iPhone and the iPhone’s battery are not being damaged in any way by using the more powerful charger. Re-charging circuitry in the iPhone is preventing it from using the extra DC power that the charger can supply.
This was tested on iPhone 4, iPhone 5, iPhone 5C, and iPhone 6.
With devices that use USB charging, the circuitry that is used to regulate and monitor the re-charging process are included in the device that is being recharged.
If the power supply is delivering less energy than the device can handle, it simply takes what it can get and re-chargers at a slower rate. If the power supply is capable of delivering more energy than the device can handle, it simply takes as much as it can handle.
Apple’s own website indicates that the iPad charger is compatible with iPhones.
iPhone 5 charging using 1 amp charger. It is drawing 5.06V at 0.94A for 4.7W of electricity. 
iPhone 5 charging using 2 amp charger. It is drawing 5.05V at 0.96A for 4.84W of electricity. 
iPad 2 charging using 1 amp charger. It is drawing 5.06V at 0.94A for 4.75W of electricity. 
iPad 2 charging using 2 amp charger. It is drawing 5.03V at 1.97A for 9.91W of electricity. 
USB Power Standards. 
USB Pin Diagram. 
