Everyone using Kamailio knows that $var() is a tricky beast. If you are not resetting it at the beginning of a script, you might get interesting effects later, or trying to use it with async statements is also not a good idea.
One of the things that tends to be overlooked is that now $null is implemented, despite its description in the documentation.
TL;DR: there is no $null, just a "0" (zero) value.
But one more thing is that an empty string is also not an empty string, but "0". So having a statement like
uac_replace_from("$var(name)", "...");
will give you actually "0" in a CallerID name if you made something like
$var(name) = "";
before. Tricky beast as is.
P.S.: To add, $vn(...) will also threat empty string as $null, but in this case will do even worse making From header like
From: <null> <sip:....>
which is just a malformed header.
