foo();
isset: boolean false
var_dump: null
$a;
foo($a);
isset: boolean false
var_dump: null
$a = 1;
foo($a);
isset: boolean true
var_dump: int 1
$a = false;
foo($a);
isset: boolean true
var_dump: boolean false
$a = null;
foo($a);
isset: boolean false
var_dump: null