Assuming $qid is a valid result of a mysql_query:

while ($r = mysql_fetch_array($qid)) {
    ...do something...
}

$r["foo"] = "bar";
echo $r["foo"];

Will not display anything.  $r["foo"] is never assigned the value "bar".  Instead, I must forcefully unset($r); or $r = array; before trying to make the $r["foo"] = assignment.

This works fine in PHP 3 but not in 4.0 beta 3.