Anwser 1:

$message is a simple variable whereas $$message is a reference variable. Example:

$user = 'bob'

is equivalent to

$holder = 'user';

$$holder = 'bob';

Anwser 2:

They are both variables. But $message is a variable with a fixed name. $$message is a variable who's name is stored in $message. For example, if $message contains "var", $$message is the same as $var.