PHP Static var not working - php

Question

I'm working on a Captcha class and i'm almost done, there is one thing that doesn't work
In the file where I put the form, I start with this line:
include 'captcha.php';
$captcha = Captcha::tryCaptcha(2,4,'#000', '#ffffff');
and this is the captch construct:
static $do_generate = TRUE;
function __construct($aantal_letters = 2, $aantal_cijfers = 4, $voorgrond = '#000000', $achtergond = '#ffffff') {
session_start();
if (self::$do_generate == TRUE) {
$letters = substr(str_shuffle('ABCDEGHJKLMNPQRSTUVWXYZ'),0 ,$aantal_letters);
$cijfers = substr(str_shuffle('23456789'),0 ,$aantal_cijfers);
$imgbreed = 18 * ($aantal_letters + $aantal_cijfers);
$_SESSION['imgbreed'] = $imgbreed;
$_SESSION['captcha'] = $letters . $cijfers;
$_SESSION['voorgrond'] = $this->hex2rgb($voorgrond);
$_SESSION['achtergond'] = $this->hex2rgb($achtergond);
}
}
so in other words I put my stuff in a session if the static var $do_generate == TRUE
So when I post the form, the captcha is getting checked by a procesor.php
like this:
if (Captcha::captcha_uitkomst() == TRUE) {
echo "Great";
} else {
echo "Wrong";
}
And this is the captcha function that checks the etered captcha code:
static function captcha_uitkomst() {
if (strcmp($_SESSION['captcha'], strtoupper(str_replace(' ', '', $_POST['captcha-invoer']))) == 0) {
return TRUE;
} else {
echo "test";
self::$do_generate = FALSE;
return FALSE;
}
}
If I enter a correct captcha code, it's all good, that works I get the echo great.
If wrong I get the echo Wrong,
Perfect, but.... when I go back to form (hit backspace one history back) to enter a correct captcha, it regenerates a new captcha.
In the class: captcha_uitkomst you see that I made the self::do_generate FALSE
And the echo 'TEST' works when it's false, (just for checking)
What am I doing wrong

Answer 1

When you hit "back", the page is reloaded. You get a new CAPTCHA.
The premise of your question is fundamentally flawed, as you have just randomly assumed that this shouldn't happen, whereas in reality this is entirely by design.
It wouldn't be a very effective CAPTCHA if you could repeatedly get it wrong then go back and try again; any bot could just start brute forcing it and learning from the experience.