Problem with dba_open() on db4 database

Hi,

I am using a db4 database to store some values (on the local 
filesystem). Here the sample code:

$handle = dba_open("/var/cache/mydb.db", "cd", "db4");
[...]
$result = dba_replace($key, serialize($data), $handle);
[...]
dba_close($handle);

This Code is used in a CLI-Script (in an Asterisk-AGI-Script) and will 
be used by some processes in parallel. The online documentation 
(http://de1.php.net/manual/en/function.dba-open.php) shows, that the 
function should "wait" if more than a process has a lock on the 
database. This works fine if I use the script an execute it in differnt 
shells in parallel.

But SOMETIMES(!) I get (in the Asterisk-Environment) the following error 
message:
dba_open(/var/cache/mydb.db,cd): Driver initialization failed for 
handler: db4: Unable to establish lock (database file already open)

So it seems, that the function "dba_open()" returns immediatly without 
waiting for lock release. But I do not know why ... ?!

I am using the following versions:
Ubuntu 12.04. LTS (up to date) with PHP 5.3.10-1ubuntu3.6 with Suhosin-Patch

Any idea?

Thank in advance
-Thorsten-