If you have an OEM charger, does the phone get hot when using it?
You know the obvious answer will be to NOT use a third party charger right?
It's normal for the phone to get get warm during charging. If the device is getting hot, with a 'real' charger, you should get the device looked at ASAP.
I have been using 3rd party chargers through my eBay guy as I test them. They have not given me any troubles. iPhone getting hot when charging? I haven't had that issue even with iPhone Apple chargers.