Detailed steps for generating verification code images using PHP and GD library

Detailed steps for generating verification code images using PHP and GD libraries

Verification code is a commonly used security verification mechanism that can be used to prevent malicious programs or automated robot operations. Automated attacks can be effectively prevented by generating a random image that requires users to enter the verification code shown in the image when logging in or registering. In this article, we will introduce in detail how to use PHP and GD library to generate verification code images.

Step 1: Install and configure the GD library
First, make sure your PHP environment has the GD library installed. If it is not installed, you can install it by editing the php.ini file or using your system's package manager.

Step 2: Create verification code generation function
Create a function in the PHP file to generate the verification code image. We will use the GD library functions to create a canvas and draw random text and distractor elements on it.

function generateCaptchaImage($width, $height, $length) {
    // 创建画布
    $image = imagecreatetruecolor($width, $height);
    
    // 定义背景色和文字颜色
    $bgColor = imagecolorallocate($image, 255, 255, 255);
    $textColor = imagecolorallocate($image, 0, 0, 0);
    
    // 填充背景色
    imagefill($image, 0, 0, $bgColor);
    
    // 生成随机字符串
    $characters = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789';
    $captcha = '';
    $strLength = strlen($characters);
    
    for ($i = 0; $i < $length; $i++) {
        $captcha .= $characters[rand(0, $strLength - 1)];
    }
    
    // 在画布上绘制随机字符串
    $fontSize = $height * 0.8;
    $fontFile = '/path/to/font.ttf'; // 替换为你自己的字体文件路径
    
    $textX = ($width - $length * $fontSize) / 2;
    $textY = ($height - $fontSize) / 2 + $fontSize;
    
    imagettftext($image, $fontSize, 0, $textX, $textY, $textColor, $fontFile, $captcha);
    
    // 添加干扰元素
    $lineColor = imagecolorallocate($image, 100, 100, 100);
    
    for ($i = 0; $i < 5; $i++) {
        imageline($image, rand(0, $width), rand(0, $height), rand(0, $width), rand(0, $height), $lineColor);
    }
    
    // 输出图像
    header('Content-Type: image/png');
    imagepng($image);
    imagedestroy($image);
    
    return $captcha;
}

Step 3: Call the verification code generation function
In your PHP file, generate the verification code image by calling the above generateCaptchaImage function, and store the verification code value in session or cookie for subsequent verification.

session_start();

$captcha = generateCaptchaImage(150, 50, 6);
$_SESSION['captcha'] = $captcha;

Step 4: Display the verification code image
In HTML, display it by specifying a PHP file in the src attribute of the <img> tag Verification code image.

<img src="captcha.php" alt="验证码">

Step 5: Verify user input
When the user submits the form, verify whether the verification code entered by the user is consistent with the stored verification code value.

session_start();

if ($_POST['captcha'] === $_SESSION['captcha']) {
    // 验证码正确
} else {
    // 验证码错误
}

Through the above steps, we successfully generated the verification code image using PHP and GD libraries, and verified the verification code through session or cookie. This can effectively prevent automated attacks and improve system security. You can customize and expand it to suit your needs, such as adding more complex distractions or using different fonts. I hope this article can help you understand the verification code generation.

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