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-- 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;

if ($sth->rows) {
    # it did the update, so the row is mine.
} else {
    # someone else took the register, let's try another one
}