Up to now you have used a fairly simple way to tell if something exists:
if (!$_POST['fred']) {
This is correct but does result in a notice/warning from the server when the value is not set. This is because you have tried to access a variable which does not exist. More correct is to use isset.
ISSET
This checks whether a variable has ever been allocated any value:
if (isset($fred)) {
If $fred exists then the test succeeds.
UNSET
Traditionally if you want to clear a variable you would set it to an empty string:
$fred="";
That is not good enough any more because ISSET will still see it (it still exists but is empty). Instead you should destroy the variable totally:
unset($fred);
