Download Directadmin's imap source:

wget http://files.directadmin.com/services/imapback-2004c1.tar.gz

Extract and move to /usr/local/imap, the Makefile has to be edited so that imap will be compiled with the -fPIC flag, find the line:

EXTRACFLAGS=-I/usr/kerberos/include -L/usr/kerberos/lib

and edit it to:

EXTRACFLAGS=-I/usr/kerberos/include -L/usr/kerberos/lib -fPIC

Next we make a symlink the kerberos to the 64bits libraries:

cd /usr/kerberos/
ln -s /usr/lib64 lib

Now we compile Imap (ignore warnings during compile time)

cd /usr/local/imap
./buildit

Now we have 64bit compiled imap libraries, next we compile PHP, go to /usr/local/directadmin/customapache/ and edit configure.php. Add the following lines

--with-imap=/usr/local/imap \
--with-imap-ssl \

Then we recompile PHP:

./build clean
./build php d

After it finishes, restart apache

service httpd restart

And remove the kerberos lib symlink to avoid compilation errors in the future:

rm /usr/kerberos/lib