字符验证码的制作分五步：
1.生成图像
2.生成随机数
3.制作干扰点、线
4.输出图像
5.销毁图像
下面直接上代码
服务器端：
<?php
session_start();
$width = 100;
$length = 30;
$captch_code = '';
$image = imagecreatetruecolor($width, $length);
$color = imagecolorallocate($image, 255, 255, 255);
imagefill($image, 0, 0, $color);
//产生随机数
for($i=0; $i<4; $i++)
{
$fontsize = 6; //?
$fontcolor = imagecolorallocate($image, rand(0,120), rand(0,120), rand(0,120));
$data = "abcdefghijklmnopqrstuvwxyz1234567890";
$fontcontent = substr($data,rand(0,strlen($data)),1);
$captch_code .= $fontcontent;
$x = ($i*$width/4) + rand($length/6,$length/3);
$y = rand($length/6,$length/3);
imagestring($image, $fontsize, $x, $y, $fontcontent, $fontcolor);
}
$_SESSION['authcode'] = $captch_code;
//干扰点
for($i=0; $i<200; $i++)
{
$pointcolor = imagecolorallocate($image, rand(50,200), rand(50,200), rand(50,200));
imagesetpixel($image, rand(1,$width-1), rand(1,$width-1), $pointcolor);
}
//干扰线
for($i=0; $i<5; $i++)
{
$linecolor = imagecolorallocate($image, rand(80,220), rand(80,220), rand(80,220));
imageline($image, rand(1,$width-1), rand(1,$width/3-1), rand(1,$width-1), rand(1,$width/3-1), $linecolor);
}
header("content-type: image/png");
imagepng($image);
imagedestroy($image);
<?php
if(isset($_REQUEST['authcode'])){
session_start();
if(strtolower($_REQUEST['authcode']) == $_SESSION['authcode']){
echo '<font color="#0000cc">输入正确</font>';
}else{
echo '<font color="#cc0000"><b>输入错误</b></font>';
}
exit();
}
?>
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<form method="post" action="./form.php">
<p>
验证码图片:<img id="captch_code" border="1" src="./captcha.php?r=<?php echo rand();?>" witdh="100px" />
<a href="javascript:void(0)" onclick="document.getElementById('captch_code').src='./captcha.php?r='+Math.random()">换一个</a>
</p>
<p>图片内容:<input type="text" name="authcode" value="" /> </p>
<p><input type="submit" value="提交" style="padding:6px 20px;"></p>
</form>
</body>
</html>
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原文地址：http://blog.csdn.net/qq_20480611/article/details/47130497
