php生成验证码函数

<?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);
?>