I finally have referrals working using the ldap_set_rebind_proc function. Don't connect to the referral server in your callback function. This is done for you. You only have to bind. The callback must return 0 if the bind succeeds or 1 if it fails. 

Consider a master - slave LDAP setup where the slave is read-only and refers writes to the master. For the PHP on the slave, you need something like this:

<?php

// Callback function
function rebind($ldap, $referral) {
    // ldap options
    ldap_set_option($ldap, LDAP_OPT_PROTOCOL_VERSION, 3);
    ldap_set_option($ldap, LDAP_OPT_REFERRALS, True);
    ldap_set_rebind_proc($ldap, 'rebind');
    // The referral is of the form:
    //  ldaps://newhost/cn=user,ou=people,dc=example,dc=com
    $refparts = explode('/', $referral);
    if (count($refparts) > 2) {
        // Get the bind dn from referral
        $dn = $refparts[3];
        // Bind to new host
        if (!ldap_bind($ldap, $dn, $pass)) {
            echo 'Could not bind to referral server';
            return 1;
        }
    } else {
        // Try anonymous bind
        if (!ldap_bind($ldap)) {
            echo 'Could not bind to referral server anonymously';
            return 1;
        }
    }
    return 0;
}
    
// Initial ldap connection to slave server
$ldap_host = 'localhost'
$ds = ldap_connect($ldap_host);
// ldap options
ldap_set_option($ds, LDAP_OPT_PROTOCOL_VERSION, 3)
ldap_set_option($ds, LDAP_OPT_REFERRALS, True)
// Set callback function
ldap_set_rebind_proc($ds, 'rebind'))
// bind
ldap_bind($ds, $dn, $pass)
// ldap write
ldap_modify($ds, $dn, $attr);

?>

Accessing passwords and other data from your callback is easier if you use a class method as the callback function. The callback would be initialized like this:

<?php

ldap_set_rebind_proc($ldap, 'MyClass::rebind');

?>