Here's a little tidbit about ldap_init() I didn't know about:

If you call ldap_set_option() to set the LDAP_OPT_AREC_EXCLUSIVE flag for the connection, ldap_init() will always connect directly to the LDAP host you specify, rather than going through the DNS SRV records. This can save a few round trips to the DNS server, which can be a big deal when connectivity to DNS isn't so hot.

See http://msdn2.microsoft.com/en-us/library/aa366938.aspx.