How to implement image captcha in PHP

How to implement image verification code in PHP

Overview:
Image verification code is a common form of verification code used to verify whether the user input is correct . When writing a website or application, we often need to use image verification codes in scenarios such as registration, login, password reset, etc. to prevent malicious attacks and automated submissions. In PHP, image verification codes can be easily implemented by using the GD library. This article will introduce how to use PHP and the GD library to generate and verify image verification codes, and provide specific code examples.

Steps:
1. Create image verification code:
First, we need to generate a random verification code string and store it in the session for subsequent verification. In the process of generating the verification code, we can use random letters, numbers or specific characters as the content of the verification code.

<?php
session_start();

$code = '';
$length = 4; // 验证码长度
$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'; // 验证码字符集

for ($i = 0; $i < $length; $i++) {
  $code .= $chars[mt_rand(0, strlen($chars) - 1)];
}

$_SESSION['captcha'] = $code; // 将验证码存储到session中

2. Generate verification code image:
Next, we can use the GD library to generate the verification code image. The GD library is a PHP extension library for image processing, which can implement some common image operations.

<?php
header("Content-type: image/png");

$imageWidth = 120; // 图像宽度
$imageHeight = 40; // 图像高度

$image = imagecreate($imageWidth, $imageHeight);
$bgColor = imagecolorallocate($image, 255, 255, 255); // 背景色为白色
$textColor = imagecolorallocate($image, 0, 0, 0); // 文本颜色为黑色

for ($i = 0; $i < strlen($code); $i++) {
  $char = substr($code, $i, 1);
  $x = $imageWidth / strlen($code) * $i + 5;
  $y = mt_rand(10, $imageHeight - 20);
  imagestring($image, 5, $x, $y, $char, $textColor);
}

imagepng($image); // 输出图像
imagedestroy($image); // 释放资源

3. Verify the verification code:
When the user submits the form, we need to verify whether the verification code entered by the user is consistent with the previously generated verification code. Verification is performed by comparing the verification code stored in the session with the verification code entered by the user.

<?php
session_start();

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
  $captcha = $_SESSION['captcha']; // 获取之前生成的验证码
  $userInput = $_POST['captcha']; // 用户输入的验证码
  if (strtolower($captcha) === strtolower($userInput)) {
    // 验证码正确
    // 执行其他操作
  } else {
    // 验证码错误
    // 给出错误提示
  }
}

Summary:
Through the above steps, we can implement image verification code in PHP. First, generate a random verification code string and store it in the session. Then, use the GD library to generate an image of the verification code. Finally, when the form is submitted, verify whether the verification code entered by the user is consistent with the previously generated verification code. By implementing image verification codes, you can increase the security of your website or application, effectively preventing malicious attacks and automated submissions.

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