Click here
<?php
error_reporting(0);
require __DIR__.'/flag.php';
$exam = 'return\''.sha1(time()).'\';';
if (!isset($_GET['flag'])) {
echo '<a href="./?flag='.$exam.'">Click here</a>';
}
else if (strlen($_GET['flag']) != strlen($exam)) {
echo 'Not allowed length';
}
else if (preg_match('/`|"|\.|\\\\|\(|\)|\[|\]|_|flag|echo|print|require|include|die|exit/is', $_GET['flag'])) {
echo 'Not allowed keyword';
}
else if (eval($_GET['flag']) === sha1($flag)) {
echo $flag;
}
else {
echo 'What\'s going on?';
}
echo '<hr>';
highlight_file(__FILE__);