This is transient and does not get preserved anywhere. Sssphhuutt!. Gone. It is not a statement, but an expression. If PHP doesn't snag on it, it will ignore it.
You were closest at the beginning when things were more explicit. One does not want to get caught up in monitoring too many variables.
Going for the meat of the matter, straight off...
$name = "Siphiwe";
echo substr($name, rand(0, strlen($name) - 1), 1);
Some people find variables to be useful; others find functions useful in place of variables.
By limiting the number of variables we declare in a particular operation, we limit the number of objects we need to monitor in a dynamic setting. This is vital. Less is more.