PHP number operation verification code

        getValidate(100,30);

    function getValidate($w,$h){
        $img = imagecreate($w,$h);

        $gray = imagecolorallocate($img,255,255,255);
        $black = imagecolorallocate($img,rand(0,200),rand(0,200),rand(0,200));
        $red = imagecolorallocate($img, 255, 0, 0);
        $white = imagecolorallocate($img, 255, 255, 255);
        $green = imagecolorallocate($img, 0, 255, 0);
        $blue = imagecolorallocate($img, 0, 0, 255);
        imagefilledrectangle($img, 0, 0, 100, 30, $black);


        for($i = 0;$i < 80;$i++){
            imagesetpixel($img, rand(0,$w), rand(0,$h), $gray);
        }


        $num1 = rand(1,99);
        $num2 = rand(1,99);

        imagestring($img, 5, 5, rand(1,10), $num1, $red);
        imagestring($img,5,30,rand(1,10),getRand(), $white);
        imagestring($img,5,45,rand(1,10),$num2, $green);
        imagestring($img,5,65,rand(1,10),"=", $blue);
        imagestring($img,5,80,rand(1,10),"?", $red);


        header("content-type:image/png");
        imagepng($img);
        imagedestroy($img);
    }
    function getRand(){
        $code = rand(0,1);
        switch ($code) {
            case 0:
                return "+";
                break;
            case 1:
                return "-";
                break;
            default:
                # code...
                break;
        }
    }

    ?>

验证码, PHP