PHP Why doesn't an unquoted array index work?

Question

PHP

Why doesn't an array index without quotes work?

$sc = array('mm', 'jj', 'dd');
define('sss', 1);
echo $sc[sss];

Should input jj, but the output is empty. What module to include or what is needed?

Answer 1

Works, of course.
codepad.org/1UzGsUua

Answer 2

Why should it work without quotes? The php syntax is...