PHP LDAP authentication via Proxy server

I have a php web system and which use AD/LDAP authentication using external directory server. I'm using this code to do the bind operation.

    ldap_set_option($conn, LDAP_OPT_PROTOCOL_VERSION, 3);
    ldap_set_option($conn, LDAP_OPT_REFERRALS, 0);
    $bind = @ldap_bind($conn, $user, $pass);

This is working as expected and able to do the authentication using port 389.

Based on the security concerns I need to do the LADP authentication (ldap_connect() and ldap_bind() functions) through proxy server.

How can I send this ldap_connect() and ldap_bind() over proxy (3128).

Need to do the ldap_bind request through proxy server. (Ex: 10.1.5.4:3128)