Changes have been made in Git for this ticket:

[jan] Fix detecting superiour objectclass' must/may attributes (Bug #9826).

  2 files changed, 18 insertions(+), 12 deletions(-)
http://git.horde.org/horde-git/-/commit/fd141fa43b966363f2c8035c62b1a4db3fcd8b36

I upgraded to PEAR package Horde_Ldap version 1.1.0
There is still a problem in class Horde_Ldap_Schema.
If there is an objectclass with no must attributes but superclasses 
would have one,
an empty array is returned.
current code:
public function must($oc, $checksup = false)
     {
         try {
             $attributes = $this->_getAttr($oc, 'must');
             if ($checksup) {
                 foreach ($this->superclass($oc) as $sup) {
                     $attributes = array_merge($attributes, $this->must($sup));
                 }
                 $attributes = array_values(array_unique($attributes));
             }
             return $attributes;
         } catch (Horde_Ldap_Exception $e) {
             return array();
         }
     }

_getAttr throws an Exception and if catched an empty arry ist 
returned, no superclass is called
Possible solution:
     public function must($oc, $checksup = false)
     {
         try {
             $attributes = $this->_getAttr($oc, 'must');
         } catch (Horde_Ldap_Exception $e) {
             $attributes =  array();
         }

         if ($checksup) {
            foreach ($this->superclass($oc) as $sup) {
                $attributes = array_merge($attributes, 
$this->must($sup, true));
            }
            $attributes = array_values(array_unique($attributes));
         }
         return $attributes;
    }

Changes have been made in Git for this ticket:

Make sure that may() and must() are recursively called (Bug #9826).

  1 files changed, 2 insertions(+), 2 deletions(-)
http://git.horde.org/horde-git/-/commit/283bceb26af9dd1fbf5ddc9cccf66a6b65859231

This is already the case.

In _checkRequiredAttributes must attributes are received with the 
checksup parameter set to false. It should default to true to get all 
required attributes. Alternativly add an parameter in turba's 
backend.php config file.

In function may() and must() superclasses are checked like:
$attributes = array_merge($attributes, $this->may($sup));

You should provide a "true" value to the parameter "checksup" to check 
ALL superclasses
(also superclasses of the superclass)
-> $attributes = array_merge($attributes, $this->may($sup,true));

Changes have been made in Git for this ticket:

[jan] Add parameter to Horde_Ldap_Schema#must() and #may() to return 
attributes from superior objectclasses too (Bug #9826).

  3 files changed, 26 insertions(+), 9 deletions(-)
http://git.horde.org/horde-git/-/commit/41119a20da71a6c2a91aa16e2a74ed721883daba

Changes have been made in Git for this ticket:

[jan] Don't throw exceptions from Horde_Ldap_Schema#must() and #may() 
(Bug #9826).

  2 files changed, 12 insertions(+), 6 deletions(-)
http://git.horde.org/horde-git/-/commit/557c7142c6a052b3c7f51f1a518d3d9d8644f603

Changes have been made in Git for this ticket:

Use Horde_Ldap_Schema to check for required attributes (Bug #9826).

  1 files changed, 20 insertions(+), 20 deletions(-)
http://git.horde.org/horde-git/-/commit/b5a1551f40db490d8c5a0a783b687be059e860a4

I don't use checksyntax, so I have appied your patch to the function
_checkRequiredAttributes

The schema is now retrieved, but:
- if an objectclass (like turbacontacts) has no MUST attribute, the
function must() in Horde_Ldap_Schema throws an Exception
I've added an empty array entry as a workaround in the parse() function for
must, may and sup attributes if the are not set yet.

- if an objectclass (like inetorgperson) has no MUST attribute but 
MUST attributes
are defined in superior objectclasses we shout recurse to get all 
required attributenames. I have implemented this (quick and dirty) 
only in the function must().
Should be done also in may().

With this changes the Turba_Driver_Ldap works now fine for me.

Does this work?

Changes have been made in Git for this ticket:

Complete half-finished conversion to Horde_Ldap_Schema (Bug #9826).

  3 files changed, 46 insertions(+), 9 deletions(-)
http://git.horde.org/horde-git/-/commit/5090bf32fab18405baf39897d30829a03511cc39

With 'checkrequired' or 'checksyntax' enables in backend configuration
turba fails.
In function _checkRequiredAttributes the schema is retrieved with
$this->_ldap->schema()

but property _ldap is undefined in the class Turba_Driver_Ldap

same behavier in  function _getSyntax