Getting Started with PHP and the GD Library: How to Create a Captcha Verification System

Getting Started with PHP and the GD Library: How to Create a Captcha Verification System

Captcha is a technology used to verify whether a user is a real human being. It is widely used in websites and applications to prevent abuse and attacks by automated scripts and bots. This article will introduce how to create a simple verification code verification system using PHP and the GD library.

The GD library is a PHP extension library for processing images. It provides some powerful image processing tools, including generating images, manipulating image pixels, and creating verification codes. Before we begin, we need to ensure that the GD library is properly installed and enabled on the server.

The first step is to generate the verification code image. We will use the functions of the GD library to create an image containing random characters. Here is a simple code example:

<?php
// 创建一个验证码图像
$image = imagecreatetruecolor(200, 50);

// 设置背景颜色为白色
$background_color = imagecolorallocate($image, 255, 255, 255);
imagefill($image, 0, 0, $background_color);

// 生成随机字符
$characters = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$code = '';
for ($i = 0; $i < 6; $i++) {
    $code .= $characters[rand(0, strlen($characters) - 1)];
}

// 将验证码保存到会话中
session_start();
$_SESSION['captcha_code'] = $code;

// 将随机字符绘制到图像上
$text_color = imagecolorallocate($image, 0, 0, 0);
imagettftext($image, 18, 0, 10, 35, $text_color, 'path/to/font.ttf', $code);

// 输出图像
header('Content-type: image/png');
imagepng($image);
imagedestroy($image);
?>

In the above code, we first create a 200x50 pixel image and then fill the background with white. Next, we generate a verification code consisting of 6 random characters and save it to the session variable. Finally, we draw the verification code onto the image and output the image to the browser in PNG format.

The second step is to create a verification system to check whether the verification code entered by the user is consistent with the generated verification code. The following is a sample code:

<?php
// 检查用户输入的验证码是否匹配
session_start();
if (isset($_SESSION['captcha_code']) && $_POST['captcha'] === $_SESSION['captcha_code']) {
    echo '验证码验证成功！';
    // 验证通过后，可以执行其他操作，如用户登录等
} else {
    echo '验证码验证失败！';
    // 验证失败后，可以执行相应的错误处理逻辑
}
?>

In the above code, we first started the session and checked whether the verification code exists in the session. We then compare the verification code entered by the user with the verification code in the session. If the two are consistent, that is, the verification is passed, and we can perform the corresponding operation (such as user login). Otherwise, the verification fails and we can execute the corresponding error handling logic.

To use the verification code verification system, we can add a verification code input field to the HTML form and compare the verification code entered by the user with the server side when submitting the form. Here is a simple sample code:

<form action="verify.php" method="post">
    <label for="captcha">验证码：</label>
    <input type="text" name="captcha" id="captcha">
    <img src="captcha.php" alt="验证码"><br>
    <input type="submit" value="提交">
</form>

In the above code, we have created a form containing a captcha input field and used captcha.php as the source of the captcha image. When a user submits a form, the form data is sent to verify.php for verification.

Through the above steps, we have successfully created a basic verification code verification system. Users need to enter the verification code correctly to pass verification. You can customize the style and length of the verification code as needed to increase system security.

Summary: This article introduces how to use PHP and GD library to create a simple verification code verification system. By creating a CAPTCHA image and comparing it to user input, we can effectively prevent abuse and attacks from automated scripts and bots. I hope this article can help you and enhance your understanding and application of the verification code verification system.

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