As root, type:
cd /usr/local/directadmin
mkdir customapache
cd customapache
wget http://files.directadmin.com/services/customapache/build
chmod 755 build
./build update
./build all
And if you want to have Zend, after the whole thing finishes, type:
./build zend
and answer all questions.
Zend install = /usr/local/Zend
php.ini = /usr/local/lib
Apache config dir = /etc/httpd/conf
php = 4.3.9
apache bin = /usr/sbin
it will try to restart the server, which might fail, so type
service httpd restart