The return array consists of the following values:
(
  [imported] => (int),
  [unchanged] => (int),
  [newuserids] => (int),
  [newsubkeys] => (int),
  [secretimported] => (int),
  [secretunchanged] => (int),
  [newsignatures] => (int),
  [skippedkeys] => (int),
  [fingerprint] => (string)
)
When invalid content is passed, all values, even skippedkeys, is 0. The fingerprint value does not exist then.
