Directadmin came with php ad CLI options. So the php scripts run as apache user.  You can use suphp , which help the php run as local user.  Dircetadmin  support suphp without much issues.  Please  use the following steps to  enable suphp in directadmin.

cd /usr/local/directadmin/custombuild/
./build clean
./build update
./build set php5_cgi yes
./build set php5_cli no
./build all d
./build rewrite_confs

Now you  installed suphp in the server

You may need to rewrite the webapps php scripts to reset the ownerships if you get internal server errors with them, eg:

./build roundcube
./build squirrelmail
./build phpmyadmin

Please make sure  to change all 777 folders to 755  permission and change all 777 php scripts to 644 permission , otherwise you will get 500 internal server error  in your wesbites.