gd库画图流程：
1，创建画布
2，创建各种颜料
3，绘画
4，保存成图片
5，销毁
html页面
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>验证</title>
<script type="text/javascript">
function update(th){
th.src=‘001.php?_s=‘+Math.random();
}
</script>
</head>
<body>
<form action=‘‘ method=‘‘>
验证码<input type=‘text‘ name=‘验证码‘/><img src=‘001.php‘ onclick=‘update(this)‘/><br/>
<input type=‘submit‘ value=‘提交‘/>
</form>
</body>
</html>
001.php生成验证码
<?php // print_r(gd_info());//判断gd2库是否开启 $rs=imagecreatetruecolor(50,30);//创建画布资源 $gray=imagecolorallocate($rs, 100, 100, 100); $fontcolor=imagecolorallocate($rs, mt_rand(150,255), mt_rand(150,255), mt_rand(150,255)); $linecolor1=imagecolorallocate($rs, mt_rand(100,200), mt_rand(100,200), mt_rand(100,200)); $linecolor2=imagecolorallocate($rs, mt_rand(100,200), mt_rand(100,200), mt_rand(100,200)); $linecolor3=imagecolorallocate($rs, mt_rand(100,200), mt_rand(100,200), mt_rand(100,200)); //填充颜色 imagefill($rs, 0, 0, $gray); //随机字符串 $str=substr(str_shuffle(‘ABCDEFGHJKMNOPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz23456789‘),0,4); //书写字符串 imagestring($rs, 5, 8, 5, $str, $fontcolor); //画干扰线 imageline($rs,0,mt_rand(0,30),50,mt_rand(0,30),$linecolor1); imageline($rs,0,mt_rand(0,30),50,mt_rand(0,30),$linecolor2); imageline($rs,0,mt_rand(0,30),50,mt_rand(0,30),$linecolor3); //输出 header(‘content-type:image/png‘); imagepng($rs); //销毁 imagedestroy($rs); ?>
原文：http://www.cnblogs.com/lzzhuany/p/4783397.html