imap.c 1.75 made an incompatible change to imap_add_body().
This is not mentioned in the CVS log or the Changelog.

Prior to PHP 3.0.14, one might say, e.g.:

$s = imap_fetchstructure($mbox, $msg);
if (!isset($s->encoding))
  echo "This is a friendly message.\n";
else echo "The body requires annoying MIME processing.\n";

As of 3.0.14, "encoding" is always set (even if zero,
i.e. ENC7BIT), "breaking" the above test.  Similar
code using the "type" property also fails.

In a perfect world, these properties would be set if and
only if Content-Type, Content-Transfer-Encoding were
explicitly present in the selected message.