array_rand() is rather useful when you want to
pick one or more random entries out of an array. It takes an
input array and an optional argument
num_req which specifies how many entries you
want to pick - if not specified, it defaults to 1.
If you are picking only one entry, array_rand() returns the key for a random entry. Otherwise, it returns an array of keys for the random entries. This is done so that you can pick random keys as well as values out of the array.
See also shuffle().