How to use captcha in PHP forms to prevent automation attacks?

How to use verification codes in PHP forms to prevent automated attacks?

With the development of the Internet, automated attacks have become a major challenge to network security. In order to protect the security of the website, many websites use verification codes for verification during the form submission process. This article will introduce how to use PHP to implement form verification codes to prevent automated attacks.

First, we need to add a verification code input box to the form page, generate a random verification code in the background, and save it in the user's Session. When the user submits the form, he or she needs to verify whether the verification code entered is consistent with the one saved in the Session. The following is a simple example:

<?php
// 生成随机验证码
function generateCaptcha() {
    $captcha = rand(1000, 9999); // 生成一个4位数的随机验证码
    $_SESSION['captcha'] = $captcha; // 将验证码保存在Session中
    return $captcha;
}

// 输出验证码到页面
function outputCaptcha() {
    $captcha = generateCaptcha();
    echo '<img src="captcha.php" alt="验证码">';
}

// 验证表单提交的验证码是否正确
function validateCaptcha($captcha) {
    if (isset($_SESSION['captcha']) && $_SESSION['captcha'] == $captcha) {
        return true;
    } else {
        return false;
    }
}
?>

<form method="POST" action="submit.php">
    <label for="name">姓名：</label>
    <input type="text" id="name" name="name" required>

    <label for="email">邮箱：</label>
    <input type="email" id="email" name="email" required>

    <label for="captcha">验证码：</label>
    <input type="number" id="captcha" name="captcha" required>

    <?php outputCaptcha(); ?>

    <button type="submit">提交</button>
</form>

In the above code, we define three functions: generateCaptcha() is used to generate a random verification code and save it to the Session, outputCaptcha() is used to output the verification code to the page, validateCaptcha() is used to verify whether the verification code entered by the user is correct.

Then, we need to create a file named captcha.php to output the verification code image. The following is a simple example:

<?php
session_start();

header('Content-Type: image/png');

$captcha = $_SESSION['captcha'];

$im = imagecreatetruecolor(100, 30);
$bgColor = imagecolorallocate($im, 255, 255, 255);
$textColor = imagecolorallocate($im, 0, 0, 0);

imagefill($im , 0 , 0 , $bgColor);
imagestring($im, 5, 10, 10, $captcha, $textColor);

imagepng($im);
imagedestroy($im);
?>

In the captcha.php file, we first use imagecreatetruecolor() to create a 100x30 pixel true color image, in the image Fill it with a white background and then use a randomly generated verification code to output black text on the image. Finally, we use imagepng() to output the image to the page.

After the user submits the form, we need to verify in the background whether the verification code entered by the user is correct. The following is an example of a simple submit.php file:

<?php
session_start();
$captcha = $_POST['captcha'];

if (validateCaptcha($captcha)) {
    // 验证码正确，继续处理表单数据
    echo '验证码正确！';
    // 处理表单数据的逻辑 ...
} else {
    // 验证码错误，返回错误提示
    echo '验证码错误！';
}
?>

With the above code, we can use verification codes in PHP forms to prevent automated attacks. Users need to manually enter the correct verification code to submit the form, which improves the security of the website. At the same time, we also ensure the uniqueness and validity of the verification code by saving it in the Session.

Of course, verification codes are only a way to prevent automated attacks and cannot completely prevent all malicious attacks. But by combining it with other security measures, we can better protect our website and reduce the risk of automated attacks.

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