找到不存在的密钥时，短三元运算符

Question

Is it legal to do something like this (without warnings):

$code = [
    0=>'code0',
    1=>'code1'
];

$key = 3;
echo $code[$key] ?: 'unknown';

I.e. using ?: instead to do:

echo isset($code[$key]) ? $code[$key] : 'unknown';

PHP > 5.3