[horde] Required IMAP functions were not found

[Maik Caro]

I got everything running on a Suse 8.
Then I copied everything to my gentoo mailserver and I get the following
error after tring to log in:
Auth_imap: Required IMAP functions were not found.

I have horde 2.1 and IMP 3.1 installed.
The test.php from horde looks good.
The tets.php from IMP returns the following error after testing the IMAP
support:
Fatal error: Call to undefined function: imap_open() in
/home/httpd/htdocs/horde/imp/test.php on line 157

Any idea