Retrieves the current thread ID. If the connection is lost, and a reconnect with mysql_ping() is executed, the thread ID will change. This means only retrieve the thread ID when needed.
The MySQL connection. If the link identifier is not specified, the last link opened by mysql_connect() is assumed. If no such link is found, it will try to create one as if mysql_connect() was called with no arguments. If by chance no connection is found or established, an E_WARNING level warning is generated.