Only set 'phpob' if $value, itself, is an object and not an array.

$value might be an array of objects. By setting phpob to false, we
are telling Horde_Pack that NONE of the values being passed are
objects. By leaving phpob unset, we let Horde_Pack make that
determination, since it is smart enough to know if an array contains
objects or not.

Fixes, among other things, fatal errors when using share caching.