$src_img=imagecreatefrompng('example.png');//源图片
    $goal_img= imagecreatetruecolor($goal_width, $goal_height);//目标图片
    imagecopyresampled($goal_img, $src_img, 0, 0, 0, 0, $goal_width, $goal_height, $img_width, $img_height);//缩放
    //输出
    header('content-type:image/png');
    imagepng($goal_img);
    imagedestroy($src_img);
    imagedestroy($goal_img);