#PHP 4.1, 88 bytes

Yeah, it is pretty long.

This assumes a default php.ini file (short_open_tag = On and register_globals = On).

<?foreach($A as$k=>$v){!$b[$v]&&$b[$v]=array($v);$b[$v][]=$k;}print_r(array_values($b));

This presents the array in an human-readable way.
The values can be passed by POST, GET and COOKIE, inside the key "A".

For a modern version, one can use (90 bytes):

<?foreach($_GET[A]as$k=>$v){if(!$b[$v])$b[$v]=[$v];$b[$v][]=$k;}print_r(array_values($b));

The result is the same, except all values have to be passed over GET parameters inside the key "A".

PHP 4.1, 88 bytes

Yeah, it is pretty long.

This assumes a default php.ini file (short_open_tag = On and register_globals = On).

<?foreach($A as$k=>$v){!$b[$v]&&$b[$v]=array($v);$b[$v][]=$k;}print_r(array_values($b));

This presents the array in an human-readable way.
The values can be passed by POST, GET and COOKIE, inside the key "A".

For a modern version, one can use (90 bytes):

<?foreach($_GET[A]as$k=>$v){if(!$b[$v])$b[$v]=[$v];$b[$v][]=$k;}print_r(array_values($b));

The result is the same, except all values have to be passed over GET parameters inside the key "A".