I have now spent enough time looking at this issue to say that ldap referrals, at least when trying to add, modify or delete a record on a slave server that correctly gives a referral to a master server, does not work in php. My suggestion is turn turn off ldap referrals and write your own add, modify and delete functions with built in referral handling. Something like this:

<?php
function ldap_referral_add($connection,$add_dn,$Add_entry,$bind_dn,$bind_pw)
    {
    $rconnection = $connection;
    $loop = 10; # max number of referral hops.
    # Turn off normal referrals
    ldap_get_option($connection,LDAP_OPT_REFERRALS,$old_referral_setting)
    do
        {
        $response = ldap_add($rconnection,$dn,$entry);
        # We get a success message:
        if ( $response ) 
            {
            ldap_unbind($rconnection);
            $loop = 0;    # Probably not needed
            ldap_set_option($connection,LDAP_OPT_REFERRALS,$old_referral_setting);
            return true;
            }
        # We get a referral message:
        elseif ( !$response && ldap_errno($rconnection) == 0x0a )
            {
            $new_server_url = $server= preg_replace('!^(ldap://[^/]+)/.*$!', '\\1', $ldap_error($rconnection)); # May need some sanity checking here
            $rconnection = ldap_connect($new_server_url);
            ldap_set_option($rconnection,LDAP_OPT_REFERRALS,0)
            ldap_set_option($rconnection,LDAP_OPT_PROTOCOL_VERSION, 3)
            ldap_bind($rconnection,$bind_dn,$bind_pw);
            $loop = $loop - 1;
            }
        else 
            {
            ldap_unbind($rconnection);
            $loop = 0;    # Probably not needed
            ldap_set_option($connection,LDAP_OPT_REFERRALS,$old_referral_setting);
            return false;
            }
        } while ( $loop > 0);
    }
?>