php 数字验证码生成

<?php
session_start();

// 生成验证码
function generateCaptcha($length = 4) {
    $characters = '0123456789';
    $captcha = '';
    for ($i = 0; $i < $length; $i++) {
        $captcha .= $characters[rand(0, strlen($characters) - 1)];
    }
    return $captcha;
}

// 生成并存储验证码
$captcha = generateCaptcha();
$_SESSION['captcha'] = $captcha;

// 输出验证码图像
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, 5, $captcha, $textColor);
imagepng($image);
imagedestroy($image);
?>