Example of generating thumbnails with php GD library

header("content-type:text/html;charset=gbk");
ini_set("date.timezone","Asia/chong");
//Determine whether the file is empty
if(empty($_FILES)){
echo "The uploaded file is too large";
exit;
}
/ /Determine whether there is an error in file upload
if($_FILES['pic']['error']){
echo "Upload file";
exit;
}
//Determine whether the file type is illegal to obtain the file suffix
$allowtype =array("jpg","png","jpeg","gif");
$a=explode('.',$_FILES['pic']['name']);
$index=count( $a)-1;
$ex=strtolower($a[$index]);
if(!in_array($ex,$allowtype)){
echo "Illegal file upload";
exit;
}
$file =date('YmdHis').rand().".".$ex;
$src=$_FILES['pic']['tmp_name'];
$des="upload/".$file;
$ rs=move_uploaded_file($src,$des);
//Thumbnail
//Read uploaded images
$image=imagecreatefromjpeg($des);
$a=getimagesize($ des);
$w=$a[0];
$h=$a[1];
if($w>$h){
$width=300;
$height=$width/$w*$ h;
}else if($w<$h){
$height=300;
$width=$height/$h*$w;
}else{
$width=300;
$height=300;
 }
//Create a blank new image
$newimage=imagecreatetruecolor($width, $height);
//Copy source image content copy new image
imagecopyresized($newimage, $image, 0,0, 0,0, $width , $height, $w, $h);
$filename="upload/s_".$file;
imagejpeg($newimage,$filename);