验证码主要用于防止恶意破解密码、刷票、论坛灌水、刷页等，日常开发中经常能使用到。如下提供了一个实用的php验证码函数，代码如下：
代码语言：javascript复制<?php
function getCode($num,$w,$h) {
$code = "";
$str='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$len=strlen($str);
for ($i = 0; $i < $num; $i ) {
$code .= $str[mt_rand(0,$len-1)];
}
//将生成的验证码写入session，备验证时用
$_SESSION["verifyCode"] = $code;
//创建图片，定义颜色值
header("Content-type: image/PNG");
$im = imagecreate($w, $h);
$black = imagecolorallocate($im, 0, 0, 0);
$gray = imagecolorallocate($im, 200, 200, 200);
$bgcolor = imagecolorallocate($im, 255, 255, 255);
//填充背景
imagefill($im, 0, 0, $gray);
//画边框
imagerectangle($im, 0, 0, $w-1, $h-1, $black);
//随机绘制两条虚线，起干扰作用
$style = array ($black,$black,$black,$black,$black,
$gray,$gray,$gray,$gray,$gray
);
imagesetstyle($im, $style);
$y1 = rand(0, $h);
$y2 = rand(0, $h);
$y3 = rand(0, $h);
$y4 = rand(0, $h);
imageline($im, 0, $y1, $w, $y3, IMG_COLOR_STYLED);
imageline($im, 0, $y2, $w, $y4, IMG_COLOR_STYLED);
//在画布上随机生成大量黑点，起干扰作用;
for ($i = 0; $i < 80; $i ) {
imagesetpixel($im, rand(0, $w), rand(0, $h), $black);
}
//将数字随机显示在画布上,字符的水平间距和位置都按一定波动范围随机生成
$strx = rand(3, 8);
for ($i = 0; $i < $num; $i ) {
$strpos = rand(1, 6);
imagestring($im, 5, $strx, $strpos, substr($code, $i, 1), $black);
$strx = rand(8, 12);
}
imagepng($im);//输出图片
imagedestroy($im);//释放图片所占内存
}
session_start();
getCode(4,60,30);
?>
