PHP上传后怎样自动修改图片的大小

function createsmallimg($dir,$source_img,$small_ex="_s") {
$img_name=substr($source_img,0,-4);
$img_ex = strtolower(substr(strrchr($source_img,"."),1));
/*注释的这段用作直接在浏览器上显示图片
$im=imagecreatefromjpeg($file);
header("Content-type: image/jpeg");
imagejpeg($im);*/
switch($img_ex){
   case "jpg":
   $src_img=ImageCreateFromJpeg($dir.$source_img);
   break;
   case "gif":
   $src_img=ImageCreateFromGif($dir.$source_img);
   break;
}
$maxheight="100";
$old_width=imagesx($src_img);
$old_height=imagesy($src_img);
if($maxheight>=$maxheight) {
   $new_height=$maxheight;
   $new_width=($new_height*$old_width)/$old_height;
}
if($img_ex=="jpg"){
$dst_img=imagecreatetruecolor($new_width,$new_height);
}else{
   $dst_img=imagecreate($new_width,$new_height);
}
ImageCopyResized($dst_img,$src_img,0,0,0,0,$new_width,$new_height,$old_width,$old_height);
$smallname=$dir.$img_name.$small_ex.".".$img_ex;
switch($img_ex) {
   case "jpg":
   ImageJpeg($dst_img,$smallname,100);
   break;
   case "gif":
   imageGif($dst_img,$smallname,100);
   break;
}
}