[PHP] if (empty versus if (isset

[John Taylor-Johnston]

What is the difference between?

if (empty... http://www.php.net/manual/en/function.empty.php "Determine 
whether a variable is empty"
and
if (isset... http://php.net/manual/en/function.isset.php "Determine if a 
variable is set and is not *|NULL|*"

I have an <input type="radio" value="something">.

If it is not checked, it is NOT empty, because it has a value, right?
But it is NOT set, right?

Is this empty, because it's value is ""?

<input type="text" value="">

Just trying to understand ... :)