Thanks! We need a test for that. PHPT test that is.

I can add a unit test, but this test will requires a GSSAPI enabled server (MS Exchange p.e.) ?
How can I ensure this ?

you can add a config.inc, and skip the test if it is not configured properly, like the pdo_mysql's tests https://github.com/php/php-src/blob/master/ext/pdo_mysql/tests/config.inc

Here is a unit test. It uses imap_include as all other imap tests, and a specific skip script.

It will be skipped on most case (need a Kerberos aware server and a not aware client)

It fails on our internal Exchange 2007 server and php 5.3.3 (RHEL-6) or 5.4.7 (Fedora 18)
It succeed with php 5.4.7+patch applied

Been butting my head against this bug for the best part of a day, thank you for reporting it, and patching it (though I can't try the patch on the production server). Worked around the problem by using a string for now.