function    test()
{
echo "hello";
}
echo "${@test()}";
echo "${@phpinfo()}";
// 原理：
$a = ‘b‘;
$b = ‘a‘;
echo $$a;

<?php
$a = ‘hello‘;
$$a = "test";
echo "${$a}";
?>  这样输出test

<?php
$a = ‘hello‘;
$$a = "test";
echo "$$a";
?>这样输出hello;