You had better post such queries on the Lonely Planet website; I will try to address it though.

The PSB are under obligation to extend your visa for such a length of time as is needed for you to legally leave China (the mainland minus Hong Kong and Macau); but if you are overstaying you will have to pay a fine.
The extension can be had if you are REGISTERED with the PSB (if you are staying at a hotel approved by the authorities to house foreigners). The registration slip automatically comes to the PSB and you will thus be "registered". If you are not staying at such a guesthouse you will probably not be given a visa extension unless you can document that you were in hospital or travelling by public means of transport!

But visa extensions become difficult to obtain once you have had your regular fill. Don't expect too much leniency! THe PSB can be quite stand-offish!