lubuntu.me/plugins/all-in-one-wp-security-and-firewall/classes/wp-security-captcha.php

<?php
class AIOWPSecurity_Captcha
{

    function __construct() 
    {
        //NOP
    }
    
    function display_captcha_form()
    {
        global $aio_wp_security;
        if($aio_wp_security->configs->get_value('aiowps_enable_bp_register_captcha') == '1' && defined('BP_VERSION')){
            //if buddy press feature active add action hook so buddy press can display our errors properly on bp registration form
            do_action( 'bp_aiowps-captcha-answer_errors' );
        }
        $cap_form = '<p class="aiowps-captcha"><label for="aiowps-captcha-answer">'.__('Please enter an answer in digits:','all-in-one-wp-security-and-firewall').'</label>';
        $cap_form .= '<div class="aiowps-captcha-equation"><strong>';
        $maths_question_output = $this->generate_maths_question();
        $cap_form .= $maths_question_output . '</strong></div></p>';
        echo $cap_form;
    }
    
    function generate_maths_question()
    {
        global $aio_wp_security;
        //For now we will only do plus, minus, multiplication
        $equation_string = '';
        $operator_type = array('&#43;', '&#8722;', '&#215;');
        
        $operand_display = array('word', 'number');
        
        //let's now generate an equation
        $operator = $operator_type[rand(0,2)];

        if($operator === '&#215;'){
            //Don't make the question too hard if multiplication
            $first_digit = rand(1,5);    
            $second_digit = rand(1,5); 
        }else{
            $first_digit = rand(1,20);
            $second_digit = rand(1,20); 
        }
        
        if($operand_display[rand(0,1)] == 'word'){
            $first_operand = $this->number_word_mapping($first_digit);
        }else{
            $first_operand = $first_digit;
        }
        
        if($operand_display[rand(0,1)] == 'word'){
            $second_operand = $this->number_word_mapping($second_digit);
        }else{
            $second_operand = $second_digit;
        }

        //Let's caluclate the result and construct the equation string
        if($operator === '&#43;')
        {
            //Addition
            $result = $first_digit+$second_digit;
            $equation_string .= $first_operand . ' ' . $operator . ' ' . $second_operand . ' = ';
        }
        else if($operator === '&#8722;')
        {
            //Subtraction
            //If we are going to be negative let's swap operands around
            if($first_digit < $second_digit){
                $equation_string .= $second_operand . ' ' . $operator . ' ' . $first_operand . ' = ';
                $result = $second_digit-$first_digit;
            }else{
                $equation_string .= $first_operand . ' ' . $operator . ' ' . $second_operand . ' = ';
                $result = $first_digit-$second_digit;
            }
        }
        elseif($operator === '&#215;')
        {
            //Multiplication
            $equation_string .= $first_operand . ' ' . $operator . ' ' . $second_operand . ' = ';
            $result = $first_digit*$second_digit;
        }
        
        //Let's encode correct answer
        $captcha_secret_string = $aio_wp_security->configs->get_value('aiowps_captcha_secret_key');
        $current_time = time();
        $enc_result = base64_encode($current_time.$captcha_secret_string.$result);
        $random_str = AIOWPSecurity_Utility::generate_alpha_numeric_random_string(10);
        AIOWPSecurity_Utility::is_multisite_install() ? set_site_transient('aiowps_captcha_string_info_'.$random_str, $enc_result, 30 * 60) : set_transient('aiowps_captcha_string_info_'.$random_str, $enc_result, 30 * 60);
        $equation_string .= '<input type="hidden" name="aiowps-captcha-string-info" id="aiowps-captcha-string-info" value="'.$random_str.'" />';
        $equation_string .= '<input type="hidden" name="aiowps-captcha-temp-string" id="aiowps-captcha-temp-string" value="'.$current_time.'" />';
        $equation_string .= '<input type="text" size="2" id="aiowps-captcha-answer" name="aiowps-captcha-answer" value="" autocomplete="off" />';
        return $equation_string;
    }
    
    function number_word_mapping($num)
    {
        $number_map = array(
            1 => __('one', 'all-in-one-wp-security-and-firewall'),
            2 => __('two', 'all-in-one-wp-security-and-firewall'),
            3 => __('three', 'all-in-one-wp-security-and-firewall'),
            4 => __('four', 'all-in-one-wp-security-and-firewall'),
            5 => __('five', 'all-in-one-wp-security-and-firewall'),
            6 => __('six', 'all-in-one-wp-security-and-firewall'),
            7 => __('seven', 'all-in-one-wp-security-and-firewall'),
            8 => __('eight', 'all-in-one-wp-security-and-firewall'),
            9 => __('nine', 'all-in-one-wp-security-and-firewall'),
            10 => __('ten', 'all-in-one-wp-security-and-firewall'),
            11 => __('eleven', 'all-in-one-wp-security-and-firewall'),
            12 => __('twelve', 'all-in-one-wp-security-and-firewall'),
            13 => __('thirteen', 'all-in-one-wp-security-and-firewall'),
            14 => __('fourteen', 'all-in-one-wp-security-and-firewall'),
            15 => __('fifteen', 'all-in-one-wp-security-and-firewall'),
            16 => __('sixteen', 'all-in-one-wp-security-and-firewall'),
            17 => __('seventeen', 'all-in-one-wp-security-and-firewall'),
            18 => __('eighteen', 'all-in-one-wp-security-and-firewall'),
            19 => __('nineteen', 'all-in-one-wp-security-and-firewall'),
            20 => __('twenty', 'all-in-one-wp-security-and-firewall'),
        ); 
        return $number_map[$num];
    }

}