most libraries for PHP are written in C.
path to follow would be dependent on your overall development style. most, if not all, companies want object oriented development, and in this case if you are going to be using MySQL, you should use the MySQLi API with a MySQL server > 4.1.
If you develop in a object oriented way, I would recommend a completely separate layer for database access, and have other items access that only. Will using a pconnect improve your site? probably not. it's more costly.