php配置 验证码错误

<?php
session_start();

function generateCaptcha() {
    $code = substr(md5(uniqid(rand(), true)), 0, 6);
    $_SESSION['captcha'] = $code;
    $image = imagecreatetruecolor(100, 30);
    $bgColor = imagecolorallocate($image, 255, 255, 255);
    imagefill($image, 0, 0, $bgColor);
    $textColor = imagecolorallocate($image, 0, 0, 0);
    imagestring($image, 5, 20, 5, $code, $textColor);
    header('Content-type: image/png');
    imagepng($image);
    imagedestroy($image);
}

function validateCaptcha($userInput) {
    if (strtolower($userInput) == strtolower($_SESSION['captcha'])) {
        return true;
    }
    return false;
}

if (isset($_GET['generate'])) {
    generateCaptcha();
} elseif (isset($_POST['captcha'])) {
    if (validateCaptcha($_POST['captcha'])) {
        echo "验证码正确";
    } else {
        echo "验证码错误";
    }
}
?>