simple captcha

<?php

if(!defined('IS_INTERNAL')) { die('Direct file connect attempt.'); }

class captcha
{
    // Default bounds of a letter image
    private static $image_default_bounds = [

        // Width
        'w' => 30,

        // Height
        'h' => 43,

        // w = (input_string_length + wp) * w
        'wp' => 1,

        // Height padding.
        'hp' => 23,
    ];

    public static function GenerateCaptcha($user_id)
    {
        // Insert variables
        $captcha_id = captcha::GenerateCapchaId();
        $captcha_creator = $user_id;
        $captcha_verification_text = cryptography::RandomString(config::GetConfig('CAPCHA_LENGTH'), false, false);
        $captcha_image = GenerateImage($captcha_verification_text);
        $captcha_entry_date = time();
        $captcha_expiry = strtotime(config::GetConfig('CAPCHA_DURATION'));

        // Creating SQL instance
        if(sql::Instance() === false) {
            throw new Exception("SQL failed");
        }

        // Is everything ok variable
        $is_ok = true;

        // Inserting query
        if($stmt = sql::Prepare("INSERT INTO `captchas` (`captcha_id`, `captcha_creator`, `captcha_verification_text`, `captcha_image`, `captcha_entry_date`, `captcha_expiry`, `enabled`) VALUES (?, ?, ?, ?, ?, ?, 'true')")) {
            $stmt->bind_param('ssssii', $captcha_id, $captcha_creator, $captcha_verification_text, $captcha_image, $captcha_entry_date, $captcha_expiry);
            $is_ok = $stmt->execute();
            $stmt->close();
        }
        else {
            throw new Exception("SQL prepare error");
        }

        if($is_ok) {
            return array(
                'success' => true,
                'response' => 'Captcha generated successfully',
                'data' => [
                    'captcha_id' => $captcha_id,
                    'captcha_creator' => $captcha_creator,
                    'captcha_image' => $captcha_image,
                    'captcha_expiry' => $captcha_expiry
                ]
            );
        }
        else {
            return array(
                'success' => false,
                'response' => 'Failed to generate Captcha.'
            );
        }
    }

    public static function VerifyCaptcha($captcha_id, $user_input)
    {
        // Creating SQL instance
        if(sql::Instance() === false) {
            throw new Exception("SQL failed");
        }

        // Is everything ok variable
        $is_ok = true;
        $is_valid = false;

        // Inserting query
        if($stmt = sql::Prepare("SELECT * FROM `captchas` WHERE `captcha_id` = ?")) {
            $stmt->bind_param('s', $captcha_id);
            $is_ok = $stmt->execute();
            $stmt->bind_result($captcha_id, $captcha_creator, $captcha_verification_text, $captcha_image, $captcha_entry_date, $captcha_expiry, $captcha_enabled);
            $stmt->store_result();
            $is_valid = $stmt->num_rows > 0;
            $stmt->fetch();
            $stmt->close();
        }
        else {
            throw new Exception("SQL prepare error");
        }

        if($is_ok) {

            // Checking row count
            if(!$is_valid) {
                return [
                    'success' => false,
                    'response' => 'No captcha ID found'
                ];
            }

            // Checking the expiry
            if($captcha_expiry < time()) {
                return [
                    'success' => false,
                    'response' => 'Captcha expired'
                ];
            }

            // Making sure the captcha is enabled.
            if(misc::IsFalse($captcha_enabled)) {
                return [
                    'success' => false,
                    'response' => 'Captcha already been used'
                ];
            }

            // Checking that the user input is valid
            if($captcha_verification_text != $user_input) {
                return [
                    'success' => false,
                    'response' => 'Invalid input'
                ];
            }

            // Disabling this captcha from working again
            if($stmt = sql::Prepare("UPDATE `captchas` SET `enabled` = 'false' WHERE `captcha_id` = ?")) {
                $stmt->bind_param('s', $captcha_id);
                $is_ok = $stmt->execute();
                $stmt->close();
            }
            else {
                throw new Exception("SQL prepare error");
            }

            if($is_ok) {
                return [
                    'success' => true,
                    'response' => 'Captcha is valid'
                ];
            }
            else {
                return [
                    'success' => false,
                    'response' => 'Internal error'
                ];
            }
        }
        else {
            return [
                'success' => false,
                'response' => 'Internal error'
            ];
        }
    }

    /**
    * Generates a captcha image from a text input.
    *
    * @param string $text The verification string that will be embeded in the image.
    */
    public static function GenerateImage($text)
    {
        // Generate bounds.
        $generated_image_bounds['w'] = ((strlen($text) + captcha::$image_default_bounds['wp']) * captcha::$image_default_bounds['w']);
        $generated_image_bounds['h'] = (captcha::$image_default_bounds['h'] + captcha::$image_default_bounds['hp']);

        // Variables to keep track of the x/y
        $generated_image_current['x'] = 0;
        $generated_image_current['y'] = 0;

        // Create image.
        $generated_image = imagecreate(
            $generated_image_bounds['w'],
            $generated_image_bounds['h']
        );

        // Going through each character.
        for ($i = 0; $i < strlen($text); $i++) {
            // Checking type of character.
            if(is_numeric($text[$i])) {
                // Number character.

                // Loading image.
                $image_file = "internal/images/".$text[$i].".png";
                $image_insert_temp = imagecreatefrompng($image_file);
            }
            else {
                // Normal character.

                // There are two images for each letter, this is getting one of the two.
                if(rand(-50, 50) >= 0) {
                    $rnd_letter_index = "1";
                }
                else {
                    $rnd_letter_index = "2";
                }

                //Loading image
                $image_file = "internal/images/".$text[$i]."-".$rnd_letter_index.".png";
                $image_insert_temp = imagecreatefrompng($image_file);
            }

            // Generating random position to insert character into.
            $generated_image_current['y'] = rand(0, $generated_image_bounds['h'] - captcha::$image_default_bounds['h']);
            $generated_image_current['x'] += (($i == 0) ? 1 : rand(captcha::$image_default_bounds['w'], captcha::$image_default_bounds['w'] + captcha::$image_default_bounds['wp']));

            while($generated_image_current['x'] > $generated_image_bounds['w']) {
                $generated_image_current['x']--;
            }

            // Inserting image to output image.
            $image_copy = imagecopy(
                $generated_image,
                $image_insert_temp,

                // dst
                $generated_image_current['x'],
                $generated_image_current['y'],

                // scr
                0,
                0,
                captcha::$image_default_bounds['w'],
                captcha::$image_default_bounds['h']
            );

            // Destroy image
            imagedestroy($image_insert_temp);
        }

        // Getting the image data
        ob_start();
        imagepng($generated_image);
        $data = ob_get_contents();
        ob_end_clean();

        // Destroy image
        imagedestroy($generated_image);

        return base64_encode($data);
    }

    /**
    * Generates a random, unqiue, capcha id.
    */
    public static function GenerateCapchaId()
    {
        $len = config::GetConfig('CAPCHA_ID_LENGTH');
        while(true) {
            $id = cryptography::RandomId($len);
            if(!captcha::DoesCapchaExist($id)) {
                break;
            }
        }
        return $id;
    }

    /**
    * Checks if a capcha id exists.
    *
    * @param string $captcha_id The ID of the capcha.
    */
    public static function DoesCapchaExist($captcha_id)
    {
        if(sql::Instance() === false) {
            throw new Exception("SQL failed");
        }

        if($stmt = sql::Prepare('SELECT COUNT(*) FROM `captchas` WHERE `captcha_id` = ?')) {
            $stmt->bind_param('s', $captcha_id);
            $stmt->execute();
            $stmt->bind_result($fetch_count);
            $stmt->fetch();
            $stmt->close();
        }
        else {
            throw new Exception("SQL prepare error");
        }

        return (isset($fetch_count) ? ($fetch_count > 0) : false);
    }
}