php random verification code php generates random verification code (graphic and text)

for($i = 0; $i<6; $i++){
    $_md_color  = imagecolorallocate($_img,mt_rand(0, 255),mt_rand(0, 255), mt_rand(0, 255));
    imageline($_img, mt_rand(0, 75),  mt_rand(0, 75),  mt_rand(0, 75),  mt_rand(0, 75),$_md_color);
}
//随机雪花
for($i = 0; $i<100;$i++){
    $_md_color = imagecolorallocate($_img, mt_rand(200, 255), mt_rand(200, 255), mt_rand(200, 255));
    imagestring($_img, 1, mt_rand(1, $_width), mt_rand(1, $_height), '*', $_md_color);
}
//黑色边框
$_black = imagecolorallocate($_img, 0, 0, 0);
imagerectangle($_img, 0, 0, $_width-1, $_height-1, $_black);
//输出验证码
for($i = 0;$i    $_mt_color = imagecolorallocate($_img, mt_rand(0, 100), mt_rand(0, 150), mt_rand(0, 200));
    imagestring($_img, mt_rand(3, 5), $i*$_width/4+mt_rand(1, 10), mt_rand(1, $_height/2), $_SESSION['code'][$i],$_mt_color);
}
header('Content-Type:image/png');
imagepng($_img);
//销毁
imagedestroy($_img);
?>