Here's a short explanation of
the configuration directives.
imap.enable_insecure_rshbool
Establishing a connection to a server may invoke rsh or
ssh commands, unless this php.ini option is disabled.
Warning
Neither PHP nor the IMAP library filter
mailbox names before passing them to rsh or ssh
commands, thus passing untrusted data to this function without disabling this
php.ini option is insecure.