I would like to issue Bitcoin coupons with a given USD value. The redeeming web app should accept the coupon pin-code and a Bitcoin address and transfer bitcoins to that address according to the current exchange rate. I'm looking for a quick / simple solution, preferably with PHP. Looking for design guidelines and any advice to help me get started.
 To be more specific:
- Is there a sample code for sending coins using bitcoind / PHP ?
- Is there a 3d party API providing such functionality (instead of using bitcoind)?
- Do MtGox and Tradehill have APIs for transfers?
- Is captcha a must?