Since USB 3.0 is rated for slightly more power (4.5 Watts vs 2.5 Watts), it’s very unlikely you’ll get problems. It will not charge faster.
That being said, USB chargers have nothing to do with USB (as in data transfer) at all. Not only do they use proprietary signalling for determining the charge current, they also provide way more power than USB (2.0 or 3.0) is meant to transfer, usually ranging from 5 to about 10 Watts.
Your phone won’t charge faster than it’s designed to. There’s an integrated charging circuit that will never draw more power than spec’d. The shipped (or official) charger will provide the best performance.
While true, this does not answer the question asked. The OP isn't interested in transfer rates but rather, plugging a USB3 cable into a USB2 charger. – ChrisInEdmonton – 2014-06-02T14:48:06.447