If you have found yourself needing to access an LDAP server from PHP and your running on a DirectAdmin box you’re going to find it is not available by default.
Here is what I needed to do in order to get it running on a Debian system:
apt-get install php5-ldap libldap-dev
Copy the ldap.so file into the PHP extensions directory:
cp /usr/lib/php5/20090626/ldap.so /usr/local/lib/php/extensions/no-debug-non-zts-20090626/
Add the extension to your active php.ini file, in my case the active php.ini file was: /usr/local/lib/php.ini. You can find out the location of your working php.ini file through phpinfo().
Restart Apache and you should be up and running (you can use phpinfo() to check if LDAP is available to you.