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-