Wrap all your operations in a single transaction and let the database do the work for you. More like:

$db->{AutoCommit}=0;
$db->begin_work();
$db->do(operation 1);
$db->do(operation 2);
$db->do(operation 3);
$db->commit();
[download]

Once you start a transaction, the database should prevent you from updating the same rows until you complete the current one.