Question

If we have this code

echo $a + 1;

php returns: Notice: Undefined variable: a in... and also 1

This is little unclear situation right? if $a is undefined (and $a is really undefined), why is undefined + 1 = 1 ?
result mus be also undefined right?
interesting to hear your opinion.

Answer 1

The default value for a non-existent variable is null. null cast to a number is 0. 0 + a number is this number.