It seems like your installation is not complete...
You can try to build PHP yourself by logging into your server as root using SSH and going to /usr/local/directadmin/customapache and running
./build clean
./build update
./build php
Then restart apache and see if the problem still exists.
Regards,
There is no /usr/local/directadmin/customapache directory.
There're conf/, custombuild/, data/ and scripts/ folders in /usr/local/directadmin/ directory