Maybe because of the new products that keeps on coming out from Apple they have launched a new program for their loyal customers worldwide. The program is called Reuse and Recycling Program wherein you can trade back your old iPhone, iPhone 3G, iPhone 3GS, iPhone 4, and many more (almost all Apple products except for iPod Touches and other stuffs) in exchange to Apple Gift Card (GC). This Apple Gift Card can be use to buy other products from the Apple Store. The program will give you the estimated value of your unit and then after you ship your device Apple will review it if it qualifies to the program and in return the company will send you the Apple Gift Card from which you can use to purchase Apple products from their Apple Store.
Unfortunately, the program is not applicable on all regions or countries. In Asia the only applicable countries that can participate to the said program are Australia, Korea, New Zealand, India, Taiwan, China, Malaysia, Hong Kong, and Singapore – Philippines is not included and the reason is for that is still unknown. You can try to contact your local retailer if there’s a similar program for you where you can trade your phone.
For more details you can visit Apple’s program.