What are the possible characters for $code
:
$randStr = "some random string that contains only: [a-zA-Z0-9_]";
$code = hash('sha512', $randStr);
SHA512 will generate 128 characters of lowercase HEX characters (0 to f), e.g.
a9046c73e00331af68917d3804f70655a9046c73e00331af68917d3804f70655a9046c73e00331af68917d3804f70655
See the documentation for hash()
for more information.