PHP7引入的"??"和"?:"的区别讲解

<?php
  $array = [
    'a' => 1,
    'b' => 2,
    'c' => [],
  ];
  $a = $array['c'] ?? 0;
  $b = $array['c'] ?: 0;
  $c = $array['d'] ?? 0;
  $d = $array['d'] ?: 0;
  $e = $array['c'] ? 1 : 0;
  $f = isset($array['c']) ? 1 : 0;
  $g = $array['d'] ? 1 : 0;
  $h = isset($array['d']['e']) ? 1 : 0;
  $i = !empty($array['c']) ? 1 : 0;
  $j = !empty($array['d']) ? 1 : 0;
  var_dump($a);
  var_dump($b);
  var_dump($c);
  var_dump($d);
  var_dump($e);
  var_dump($f);
  var_dump($g);
  var_dump($h);
  var_dump($i);
  var_dump($j);