php 生成验证码

public function getyCodeAction() {     //生成验证码
    $num = '4';
    $w = '60';
    $h = '34';

    $code = "";
    for ($i = 0; $i < $num; $i++) {
        $code .= rand(0, 9);
    }

    //4位验证码也可以用rand(1000,9999)直接生成
    //将生成的验证码写入session，备验证时用
    $_SESSION["helloweba_num"] = $code;
    //创建图片，定义颜色值
    ob_clean();
    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
    return 0;
}