My understanding is that squirrelmail does not demand usage of php-imap
but will exhibit better performance if it does use it.
With respect to php-imap, it does depend on and build against the
c-client library that was part of the UW imap-devel package. I believe
those who have pointed out that php-imap is disabled in FC2 are correct.
To support php-imap, if its really needed, would require an rpm with
c-client libs in it.