$filename = "./QR/$id.jpg";

$percent = 0.4;

list($width, $height) = getimagesize($filename);

$new_width = $width * $percent;

$new_height = $height * $percent;

$image_p = imagecreatetruecolor($new_width, $new_height);

$image = imagecreatefromjpeg($filename);

imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);

imagejpeg($image_p,'./images/thumb/'.$id.'.jpg');

filename为文件名；

percent为缩小比例；

imagejpeg为输出保存图片。