I've dealt with a similar problem once...

The key to the solution is that the $sth->rows method will return the number of rows affected by the last query-like statement. SELECT *and* UPDATE are query-like statements, so here it goes...

You have to create a control field in the table with an int value and set it's default value to some non-null value.

Then you do the following...

-- fetch 10 to avoid querying too many times....
SELECT * FROM tablex ORDER BY dateofinsertion LIMIT 10;
-- Mark the register as owned by you
UPDATE tablex SET controlfield=controlfield+1 WHERE id=$id AND control
+field=$controlfieldvalue;
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When you execute the second query, you test the rows.

if ($sth->rows) {
    # it did the update, so the row is mine.
} else {
    # someone else took the register, let's try another one
}
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Simple, isn't it? If some process fail, next time a process get the 10 registers, it will get that again and try to process.