<pre>
The summary says it all, I guess.  

$indexed = array();
$assoc = array();
$nullassoc = array();
while($row = ibase_fetch_row($sth)) {
       array_push($indexed, $row[0]);
       $assoc[$row[0]] = $row[1];
        $nullassoc["${row[0]}\0"] = $row[1];
}
 
foreach($indexed as $key=>$val) {
   echo "indexed key: '$key' val: '$val'<br>\n";
}
 
foreach($assoc as $key=>$val) {
   echo "assoc key: '$key' val: '$val'<br>\n";
}
 
foreach($nullassoc as $key=>$val) {
   echo "nullassoc key: '$key' val: '$val'<br>\n";
}
If you run this code with a select statement such as
select varcharcol, varcharcol2 from table, you should see the problem. You may need to run it a couple times to actually see it happen.

The problem could also be in the routine that handles 
$array[$key] = $val, as evidenced by the first runthrough,
where the values appear to be correctly null terminated.

Feel free to contact me with any questions or if this doesn't make sense!

Thanks
 
</pre>