Use ldap_bind()
http://www.php.net/manual/en/function.ldap-bind.php
// using ldap bind
$ldaprdn = 'id=username,dc=server,dc=com'; // ldap rdn or dn
$ldappass = 'password'; // associated password
// connect to ldap server
$ldapconn = ldap_connect("ldap.example.com")
or...