title: PHP Captcha without session
body: <p>Ok, here is an issue: in the project i'm working on, we can't rely on server-side sessions for any functionality. </p>
  
  <p>The problem is that common captcha solutions from preventing robotic submits require session to store the string to match captcha against.</p>
  
  <p>The question is - is there any way to solve the problem without using sessions? What comes to my mind - is serving hidden form field, containing some hash, along with captcha input field, so that server then can match these two values together. But how can we make this method secure, so that it couldn't be used to break captcha easily.</p>