解题

进来让我猜字符，然后点击Source，得到源码

<?php
include 'config.php'; // FLAG is defined in config.php

if (preg_match('/config\.php\/*$/i', $_SERVER['PHP_SELF'])) {
  exit("I don't know what you are thinking, but I won't let you read it :)");
}

if (isset($_GET['source'])) {
  highlight_file(basename($_SERVER['PHP_SELF']));
  exit();
}

$secret = bin2hex(random_bytes(64));
if (isset($_POST['guess'])) {
  $guess = (string) $_POST['guess'];
  if (hash_equals($secret, $guess)) {
    $message = 'Congratulations! The flag is: ' . FLAG;
  } else {
    $message = 'Wrong.';
  }
}

代码审计
提示flag在config.php，第一个if是过滤config.php，看的wp，只要在后面加入一个ascii码不可见字符就可以绕过
第二个if是get请求中有source就显示源码
第三个if是POST提交guess，去才字符串，实际没有用

任取一个空白字符