Weak typing in PHP: why use isset at all?

Question

It seems like my code works to check for null if I do

if ($tx) 

or

if (isset($tx))

why would I do the se

Answer 1

It also important to note that a string in PHP could be set but empty. So the isset would return true even though there is nothing in the string, to prevent this I use a function something like this to replace isset: