成功回傳TRUE失敗回傳FALSE。
ldap_close() closes the link to
the LDAP server that's associated with the specified link_identifier.
This call is internally identical to
ldap_unbind(). The LDAP API uses the call
ldap_unbind(), so perhaps you should use this in
preference to ldap_close().
注: This function is an alias of ldap_unbind().