If your iPhone shows a battery symbol when you turn it on and then gets stuck at the apple logo there is nothing more frustrating than that. Because in that case your iPhone will not even be detected by iTunes for you to update it to the latest firmware.
This usually happens if your iPhone was left somewhere and the battery got discharged to a level that it had to turn itself off. That is why you see the battery symbol.
To solve this problem you need to follow these steps. These steps are DIFFERENT FROM DFU mode.
1) If your iPhone is connected to the computer disconnect it.
2) Hold the power and home button together and keep holding it. You will see the apple logo and then after some time the screen will go black again. This might take 20 seconds to 5 minutes but do not lose patience. When it comes back you will see the iTunes logo.
3) If you know how to jailbreak it go ahead, if you do not want to jailbreak just restore it and you will have the latest version of iPhone firmware.
I hope this will be helpful to some people. If your iPhone is rather stuck on iTunes logo try steps to recover from iTunes logo.