可变变量输出变量值

https://www.php.net/manual/zh/language.variables.variable.php
例题：

• bugku-冬至红包：https://blog.csdn.net/qq_49422880/article/details/117428095

变变量绕过 sha1 绝对等于校验

• [极客大挑战 2019]Havefun：传参

  <?php
  $a = "skye";
  $$a = "china";//$skye="china";
  echo $a;
  // skye
  echo $$a;//echo $skye;
  // china