怎么把验证码的图片存起来

如代码GD生成了图片$im:

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   public function generateCode(){
        $w = 50;
        $h = 20;
        $this->load->config('common');
        $common_config = $this->config->item('common');
        $pic_verify_code = $common_config['pic_verify_code'];
        $code = func::getRandCode($pic_verify_code);
        //创建图片，定义颜色值
        $im = imagecreate($w, $h);
        $black = imagecolorallocate($im,90,90,90);
        $gray = imagecolorallocate($im, 189,252,201);
        $bgcolor = imagecolorallocate($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 < $pic_verify_code; $i++) {
            $strpos = rand(1, 6);
            imagestring($im, 5, $strx, $strpos, substr($code, $i, 1), $black);
            $strx += rand(8, 12);
        }

那么问题来了，怎么把这个图片存到我的服务器上面呢?

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imagepng($im,'pic.png');

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验证码直接发给客户端不就好了，保存起来干嘛；
xxx.php直接：
imagepng($im);
登陆那里<img src="xxx.php" />

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write to file ,e. fwite()