php 数字加字母验证码

<?php
session_start();

// 生成验证码
function generateCaptcha($length = 6) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $captcha = '';
    for ($i = 0; $i < $length; $i++) {
        $captcha .= $characters[rand(0, strlen($characters) - 1)];
    }
    $_SESSION['captcha'] = $captcha;
    return $captcha;
}

// 显示验证码
echo generateCaptcha();

// 验证用户输入
if (isset($_POST['captcha'])) {
    if ($_POST['captcha'] == $_SESSION['captcha']) {
        echo '验证码正确';
    } else {
        echo '验证码错误';
    }
}
?>

<!-- HTML部分 -->
<form method="post">
    <img src="captcha.php" alt="CAPTCHA Image" />
    <input type="text" name="captcha" required />
    <input type="submit" value="提交" />
</form>