이미지 기반 CAPTCHA 생성 클래스를 구현하여 웹 폼 보안을 강화할 수 있습니다. GD 라이브러리를 활용한 PHP 구현 예시입니다.

CAPTCHA 클래스 구조

<?php
session_start();
class CaptchaGenerator {
    private $charSet = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz23456789';
    private $captchaText;
    private $textLength = 4;
    private $imgWidth = 140;
    private $imgHeight = 45;
    private $image;
    private $fontFile = './arial.ttf';
    private $fontSize = 18;
    
    public function __construct() {
        // 폰트 경로 설정 필요
    }
    
    private function generateText() {
        $maxIndex = strlen($this->charSet) - 1;
        $this->captchaText = '';
        for ($i = 0; $i < $this->textLength; $i++) {
            $this->captchaText .= $this->charSet[rand(0, $maxIndex)];
        }
    }
    
    private function createBackground() {
        $this->image = imagecreatetruecolor($this->imgWidth, $this->imgHeight);
        $bgColor = imagecolorallocate($this->image, rand(180, 255), rand(180, 255), rand(180, 255));
        imagefilledrectangle($this->image, 0, 0, $this->imgWidth, $this->imgHeight, $bgColor);
    }
    
    private function drawText() {
        $charSpacing = $this->imgWidth / $this->textLength;
        for ($i = 0; $i < $this->textLength; $i++) {
            $textColor = imagecolorallocate($this->image, rand(0, 120), rand(0, 120), rand(0, 120));
            $xPos = $i * $charSpacing + rand(3, 8);
            $yPos = $this->imgHeight * 0.7;
            imagettftext(
                $this->image, 
                $this->fontSize, 
                rand(-20, 20), 
                $xPos, 
                $yPos, 
                $textColor, 
                $this->fontFile, 
                $this->captchaText[$i]
            );
        }
    }
    
    private function addNoise() {
        for ($i = 0; $i < 5; $i++) {
            $lineColor = imagecolorallocate($this->image, rand(0, 150), rand(0, 150), rand(0, 150));
            imageline(
                $this->image,
                rand(0, $this->imgWidth),
                rand(0, $this->imgHeight),
                rand(0, $this->imgWidth),
                rand(0, $this->imgHeight),
                $lineColor
            );
        }
        for ($j = 0; $j < 70; $j++) {
            $pixelColor = imagecolorallocate($this->image, rand(150, 250), rand(150, 250), rand(150, 250));
            imagesetpixel(
                $this->image,
                rand(0, $this->imgWidth),
                rand(0, $this->imgHeight),
                $pixelColor
            );
        }
    }
    
    private function outputImage() {
        header('Content-Type: image/png');
        imagepng($this->image);
        imagedestroy($this->image);
    }
    
    public function render() {
        $this->createBackground();
        $this->generateText();
        $this->drawText();
        $this->addNoise();
        $this->outputImage();
    }
    
    public function getText() {
        return strtolower($this->captchaText);
    }
}

주의: 실제 사용 시 폰트 파일 경로를 정확히 설정해야 합니다.

CAPTCHA 생성 스크립트

<?php
session_start();
require 'CaptchaGenerator.php';
$generator = new CaptchaGenerator();
$generator->render();
$_SESSION['captcha_code'] = $generator->getText();

HTML에서 사용법

<img src="captcha.php" 
     alt="CAPTCHA" 
     onclick="this.src='captcha.php?r='+Math.random()">