I suspect there's something else going wrong here in code that you haven't posted. There's no reason ++ wouldn't increment beyond 10. Try printing or checking the value of $sessionid right after you issue the ++ ... my stab in the dark is that somewhere else in your code you're accidentally setting $sessionid to 10

BTW, you might try this more "perlish" way of writing the first two lines:

$sessionid = $logquery->fetchrow_array() || 0;
$sessionid++;
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HTH