Why check both isset() and !empty()

Question

Is there a difference between isset and !empty. If I do this double boolean check, is it correct this way or redundant? and is there a shorter way

Answer 1

$a = 0;
if (isset($a)) { //$a is set because it has some value ,eg:0
    echo '$a has value';
}
if (!empty($a)) { //$a is empty because it has value 0
    echo '$a is not empty';
} else {
    echo '$a is empty';
}