Mutual authentication with ldap_start_tls()

Hi php-general, I am trying to make my web server (Apache 2 + mod_ssl) connect to my ldap server with mutual authentication using PHP. This means I want them to exchange their certificate. For this purpose, I connect to ldap on port 389 then start TLS using ldap_start_tls(). This works perfectly as long as I do not make my ldap server require client certificate. It seems to me that the web server is not able to send its certificate. I did not find any documentation on the use of TLS with PHP. I do not even know what is used to validate my ldap server certificate! If anyone knows how to help (documentations, ideas, tips,...), I would be really grateful Thanks Steph