mysql_pconnect() works most likely mysql_connect() with two major differences.
First, at the time of connection, the function would try to find a persistent link that’s already open with the same server, username and password. If one is found, an identifier for it will be returned instead of opening a new connection.
Second major differences is that the connection to the SQL server will not be closed at the execution of the script ends. Instead, the link will remain open for future use and the mysql_close() will not close links opned by mysql_pconnect().
PDO PHP Data Objects.
PDO is stand for PHP Data Objects. PDO is a lean, the major adavtage of PDO is you can connect any allowe database via php. Now developers can write portable code much easier. PDO is not an abstraction layer like PearDB. PDO is a more like a data access layer which uses a unified API (Application Programming Interface).
PHP 5 MySQLi – More Imprpoved database driver. The MySQLi Extension is a relational database driver that is used to connect to any mysql database in the PHP programming language.