PHP generates verification code picture study notes_PHP tutorial

header("Content-type:image/png");
set_time_limit(0);//设置PHP超时时间
$url = $_GET['url'];
$url = "http://vcer.baidu.com/verify";
if(empty($url)){
 echo "没有图片";
 die;
}
$imginfo = GetImageSize ( $url );  
$type = exif_imagetype($url);
$imgw = $imginfo [0];  
$imgh = $imginfo [1];
$bg = imagecreatetruecolor($imgw,$imgh);
if($type==IMAGETYPE_GIF){
 $image = imagecreatefromgif($url);
}elseif($type==IMAGETYPE_JPEG){
 $image = imagecreatefromjpeg($url);
}elseif($type==IMAGETYPE_PNG){
 $image = imagecreatefrompng($url);
}
 
imagecolorallocate($image,255,255,255);
imagecopy($bg,$image,0,0, 0,0,$imgw,$imgh);
imagedestroy($image);
ImagePng($bg);