I discoverd the following:

1. A query is performed using a Sybase function
2. fetch_array() is beeing used
2. data set contains a field wih a unix timestamp, that needs to be converted by date()
3. if I use the field index (e.g. $row[1]) everything is fine
4. ifI use the field name (e.g. $row["date"]) and use the date() on this variable, the function isn't executed all the time. On some data-sets it still shows the original timestamp instead of the formated data.

Here is an example code:

$row = sybase_fetch_array($result);
$row [1] = date("d.m.y H:i",$row [1]);<- works fine
$row["datum"] = date("d.m. H:i",$row["datum"]);<- works sometimes

Hope this helps.
Regards
Torsten