Easy Cookie [by ICS]
The many tutorials online on howto implement the CAPTCHA are very confusing and opt for a long way to implement this filter by mostly recommending the usage of PHP-Sessions to match a larger browsers' settings [users].
I believe that nowdays most of Users set their Browsers to accept Cookies, this system is Cookie-based.
...If you feel necessary, warn your Users to accept Cookies in order to enable its usage.
Hereafter is a form to which we want to add a scrambled image filtering, this will discourage easy going spammers and Users must be human, not Spam-Programs [bots] :

Place holder for routine form input fields →  User will type the scrambled ChArS herein →  let's draw the scrambled image into this FORM ↓ ←   [<img src=human.php border=0>] ...leave the Other_POST_Variables = Post_VARS Fill in the ChArS and Submit for testing :-)	←	<form method=POST action=$PHP_SELF> <input type=text name='Other_POST_Variables'value='Post_VARS'> <input type=text name='cookie_name'autocomplete='off'> <img src=human.php border=0> <input type=submit value=Submit> </form> Data-Flow : By clicking on Submit , the cookie_name's value will be sent. . . . let's then compare the User-Input Value with ccs' Value we set if($cookie_name=="$ccs" && $ccs!="") {echo"<script language=JavaScript>alert('$ccs');</script>';} ...depending on the response of the above conditional statement, the data-flow can be adjusted as desired after this checking point.