php后台验证码错误

<?php
session_start();

// 生成验证码
if (empty($_SESSION['captcha'])) {
    $_SESSION['captcha'] = substr(md5(uniqid(rand(), true)), 0, 6);
}

// 显示验证码图片
header('Content-type: image/png');
$image = imagecreatetruecolor(100, 30);
$bgColor = imagecolorallocate($image, 255, 255, 255);
$textColor = imagecolorallocate($image, 0, 0, 0);
imagefilledrectangle($image, 0, 0, 100, 30, $bgColor);
imagestring($image, 5, 20, 8, $_SESSION['captcha'], $textColor);
imagepng($image);
imagedestroy($image);

// 验证验证码
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    if (isset($_POST['captcha']) && $_POST['captcha'] == $_SESSION['captcha']) {
        echo "验证码正确";
    } else {
        echo "验证码错误";
    }
}
?>