locking dmb files....how to?

[Anon]

Hello all!
I'have read tons of posts that say that dba_open() open and locks the file at the same time, but I can't say that:

Please try this

First process:

tries to fetch the key "firstkey" thinking the value is "Good morning!".

$id = dba_open ("mydba.db", "r", "db3");

if (!$id) {
die("dba_open failed\n") ;
exit;
}

sleep(10) ;

$data=dba_fetch("firstkey",$id) or die("Cant'fetch") ; ;

dba_close($id) ;

echo($data) ;

---

Second process:

While "mydba.db" is open from first process

second process opens the file too, and replaces the key "firstkey" with value "Good evening!".

Result:

First process reads "Good evening!".....

Can anybody tell me how to lock a dba file in a reliable way,please?

Thanks