PHP 簡易實用驗證碼類 (GD 庫)

封裝一個可配置的 Captcha 類，支援：寬高、字元數、Session 鍵名、隨機背景色、干擾像素、字元隨機位置/大小/顏色。使用極簡：new Captcha()->create() 直接輸出圖片。

完整類代碼

class Captcha {
    private $width = 80, $height = 20, $charNum = 4;
    public $code;           // 生成的驗證碼
    private $img;           // 圖片資源
    private $sessionKey = 'captcha_code';

    public function __construct() {
        if (session_status() === PHP_SESSION_NONE) session_start();
    }

    public function config($width = 80, $height = 20, $charNum = 4, $sessionKey = 'captcha_code') {
        $this->width = $width;
        $this->height = $height;
        $this->charNum = $charNum;
        $this->sessionKey = $sessionKey;
        return $this; // 鏈式調用
    }

    public function create() {
        header('Content-Type: image/png');
        $this->generateCode();
        $this->createImage();
        $this->drawDisturb();
        $this->drawCode();
        imagepng($this->img);
        imagedestroy($this->img);
        $_SESSION[$this->sessionKey] = $this->code;
    }

    private function generateCode() {
        $this->code = strtoupper(substr(md5(rand()), 0, $this->charNum));
    }

    private function createImage() {
        $this->img = imagecreate($this->width, $this->height);
        $white = imagecolorallocate($this->img, 255, 255, 255);
        $black = imagecolorallocate($this->img, 0, 0, 0);
        imagefilledrectangle($this->img, 0, 0, $this->width-1, $this->height-1, $white);
        imagerectangle($this->img, 0, 0, $this->width-1, $this->height-1, $black);
    }

    private function drawDisturb() {
        for ($i = 0; $i < 200; $i++) {
            $color = imagecolorallocate($this->img, rand(0,255), rand(0,255), rand(0,255));
            imagesetpixel($this->img, rand(2, $this->width-2), rand(2, $this->height-2), $color);
        }
    }

    private function drawCode() {
        $charW = $this->width / $this->charNum;
        for ($i = 0; $i < $this->charNum; $i++) {
            $color = imagecolorallocate($this->img, rand(0,255), rand(0,128), rand(0,255));
            $x = $charW * $i + rand(2, 5);
            $y = rand(0, $this->height - 15);
            imagechar($this->img, rand(5, 8), $x, $y, $this->code[$i], $color);
        }
    }
}

使用範例

// captcha.php (單獨檔案)
require 'Captcha.php';
new Captcha()->config(100, 30, 4)->create();

<!-- 表單頁面 -->
<img src="captcha.php" id="captchaImg" onclick="this.src='captcha.php?'+Date.now()" style="cursor:pointer">
<input name="captcha" placeholder="點擊圖片刷新">

// 驗證邏輯
session_start();
if (strtoupper($_POST['captcha']) !== $_SESSION['captcha_code']) {
    die('驗證碼錯誤');
}
// 通過後清除，防重放
unset($_SESSION['captcha_code']);

安全與體驗增強建議

• 字元集擴充：排除易混淆字元 (0/O, 1/I/l, 2/Z, 5/S, 8/B) 組成字元池。
• TrueType 字體：imagettftext() + 多款字體 + 旋轉角度 + 扭曲，大幅提高 OCR 識別難度。
• 點擊/滑塊驗證：前端行為分析 (Google reCAPTCHA v3、Geetest、滑塊拼圖) 替代純圖形驗證碼。
• Session 安全：session_regenerate_id(true) 登入後重生成 ID，設定 session.cookie_httponly=1、session.cookie_secure=1 (HTTPS)。