You will need to get and compile LDAP client libraries from either the University of Michigan ldap-3.3 package or the Netscape Directory SDK 3.0 to compile PHP with LDAP support.
In the same way as there are no hard rules about how you organise the directory structure of a hard disk, a directory server manager can set up any structure that is meaningful for the purpose. However, there are some conventions that are used. The message is that you can not write code to access a directory server unless you know something about its structure, any more than you can use a database without some knowledge of what is available.