I had a similar situation, only I was trying to make sale ID's for
purchases.  Here is the code I used:

$query25="lock tables saleID write";
$mysql_result25=mysql_query($query25, $mysql_link);
$query26="select * from saleID";
$mysql_result26=mysql_query($query26, $mysql_link);
$query27="update saleID set SaleID = SaleID + 1";
$mysql_result27=mysql_query($query27, $mysql_link);
$query28="unlock tables";
$mysql_result28=mysql_query($query28, $mysql_link);

Hope this helps
-Angela Curtis

>          database collision?
>    Date:
>          Tue, 9 Jan 2001 11:45:58 +0100
>   From:
>          "Luescher Samuel (MMVZSLU)"
>      To:
> hi there. i am developing a database app to manage dynamic sites. in a

> nutshell, i have an item table (to store all the content) and a
> table (to register who's allowed to edit/view specific items).
> now, when creating a new item, i do the following things:
> - determine a new permission id (which is the permission table primary

> key, kinda "SELECT MAX(id) FROM permission_table" and then increase
> result by one. i don't use AUTO_INCREMENT columns on purpose.)
> - create an entry in the permission table
> - create an entry in the item table, including the permission id as
> relational attribute
> now, my question is: since there may be multiple php processes
running, if
> two users simultaneously create an item and post it at the same moment
> couldn't it happen that the process of user#1 has already determined
> permission id, while user#2 determines the SAME id, creates the entry
> user#1 will get an error because the item id was already taken in the
> meantime? what can i do to avoid such security/integrity holes?
> thanks for listening - sam

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