archived-web-php/manual/spam_challenge.php

<?php
// simple and stupid SPAM protection (using little challenges)

const NUMS = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'];

function plus($a, $b) {
    return $a + $b;
}

function gen_plus($a) {
    return mt_rand(0, 9 - $a);
}

function minus($a, $b) {
    return $a - $b;
}

function gen_minus($a) {
    return mt_rand(0, $a);
}

function print_infix($name, $a, $b) {
    return "$a $name $b";
}

function print_prefix($name, $a, $b) {
    return "$name($a, $b)";
}

const CHALLENGES = [
    // name, print, generator
    ['max',   'print_prefix'],
    ['min',   'print_prefix'],
    ['minus', 'print_infix', 'gen_minus'],
    ['plus',  'print_infix', 'gen_plus'],
];

// generate a challenge
function gen_challenge() {
    $c = CHALLENGES[mt_rand(0, count(CHALLENGES) - 1)];

    $a = mt_rand(0, 9);
    $an = NUMS[$a];
    $b = isset($c[2]) ? $c[2]($a) : mt_rand(0, 9);
    $bn = NUMS[$b];

    return [$c[0], $an, $bn, $c[1]($c[0], $an, $bn)];
}

// test an answer for validity
function test_answer($name, $an, $bn, $answer) {
    foreach (CHALLENGES as $x) {
        if ($x[0] === $name) {
            $c = $x;
            break;
        }
    }

    $a = array_search($an, NUMS, false);
    $b = array_search($bn, NUMS, false);

    if (empty($c) || $a === false || $b === false) return false;

    return (NUMS[$c[0]($a, $b)] === $answer);
}