PHP7.0新增特性:null合并运算符 ?? 和 ?:

//?? 等价于 isset
$a = '';
$b = isset($a) ? $a : 'hahha';
$b = $a ?? 'hahha';
//$b is ''
//适用于多层级array的属性读取
$arr = [ 'son'=>['name'=>'child'] ];
echo $arr['son']['age'] ?? '19';
//output: 19
echo $arr['non']['name'] ?? 'non child';
//output: non child

//?: 等价于empty
$c = empty($a) ? 'hahaha' : $a;
$c = $a ?: 'hahaha';
//c is 'hahaha'