如果成功则返回 TRUE，失败则返回 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().