sub unique_id() {
    # Use Apache's mod_unique_id if available
    return $ENV{UNIQUE_ID} if exists $ENV{UNIQUE_ID};

    require Digest::MD5;

    my $md5 = new Digest::MD5;
    my $remote = $ENV{REMOTE_ADDR} . $ENV{REMOTE_PORT};

    # ** Note ** This is intended to be unique, not unguessable
    my $id = $md5->md5_base64(time, $$, $remote);
    $id =~ tr|+/=|-_.|;     # make non-word characters URL friendly
    return $id;
}