60

Let's see the code right away...

We can see that 1. PHPSESSID must be numeric, 2. We need to somehow acess ./readme/{$_SESSION['idx']}.txt before it's erased.

First I changed the cookie value by 1 and saw that it created the file and then erose it after a second.

I kept tried to access /readme/48895.txt, yet it didn't work. I searched a bit and realized that they must be in different session for it to work. I thought the different tap is equal to diffrent session. Now I know that session is dependent on browser, so I chose two different browsers (chromium, chrome) to access it.

now, the file was accessible in different browser.

What we need is not the content of the brower but to invoke solve(60) so I acessed same page in different brower in less than a second