Connecting to a MySQL server often involves providing hostnames, usernames and passwords. Use a .my.cnf configuration file to provide defaults that simplify working with a MySQL server.
Providing a default password reduces security. Take effort to make sure that the password cannot be read by other users on the system. If the server runs locally, use credentials that are only allowed to connect locally.
This picks $num random keys from $arr (array_rand), flips keys with values (array_flip), intersects the picked keys with those of $arr (array_intersect_keys) and returns the corresponding elements of $arr.