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.