The pilot placed a gasoline powered "weed whip" on the rear passenger seats of the airplane for a personal flight. The pilot smelled a "strong" smell of gasoline as the airplane touched down for landing at the destination airport. During the after landing taxi, the engine end of the weed whip fell behind the two front seats. The pilot stated that he reached back and placed the weed whip back onto the seats and "something caused it to ignite." The pilot stated that he was charging his cell phone where the weed whip was located. The pilot stated that the smoke was so bad that he pulled the mixture control to idle cutoff and exited the airplane. The pilot was uninjured. The airplane was then consumed by fire.