takinng same value users

hi,

I'm using phpr5.1(2135)
my table is
sno int(9) auto_increment,

evrak_no int(9),

..

..
----
I have 3 user and this user same unit(xx)

and this users must be same unit(xx)

--
In my project

function BeforeAdd($values,$message,$inline)

{
$str = "select max(evrak_no) from gelen_evrak where unit=xx";

$rs = db_query($str,$conn);

$data = db_fetch_numarray($rs);

$values["evrak_no"]=$data[0]+1
..

}

My problem is,
While each user must take different numbers,but sometimes the same value for each user.
thnx for solutions