Undefined variables == Undefined behaviour.

Here's a nice one for a PHP exam. Given this complete code example, will it print 'aa' or 'bb'?

<?php

if ($variable != 0) {
  print('aa');
} else {
  print('bb');
}

?>

Let's apply the rules of logic here. $variable is unset, so it is not '0'. Therefor it should print 'aa'.

Wrong.

Let's apply the rules of PHP here. PHP will BLACK BOX MAGIC VOODOO and from that it naturally follows that PHP will print 'bb':

[todsah@host]~$ php ./magic_voodoo.php

Notice: Undefined variable: variable in /home/todsah/magic_voodoo.php on line 3
bb

And that, dear PHP developers, is why warnings and notices in PHP should be removed in favour of errors. At. All. Times. I hate PHP.