/**
 * 根据比例计算图片缩放尺寸函数
 *
 * @param string $img 图片路径
 * @param int $max_w 最大缩放宽
 * @param int $max_h 最大缩放高
 * @return array
 * @author 512-笔记 jb51.cc
 */
function chImageSize($img,$max_w,$max_h) {
 $size = @getimagesize($img);
 $w = $size[0];
 $h = $size[1];

 //计算缩放比例
 @$w_ratio = $max_w / $w;
 @$h_ratio = $max_h / $h;

 //决定处理后的图片宽和高
 if (($w <= $max_w) && ($h <= $max_h)) {
  $tn['w'] = $w;
  $tn['h'] = $h;
 } else if (($w_ratio * $h) < $max_h) {
  $tn['h'] = ceil($w_ratio * $h);
  $tn['w'] = $max_w;
 } else {
  $tn['w'] = ceil($h_ratio * $w);
  $tn['h'] = $max_h;
 }
 $tn['rc_w'] = $w;
 $tn['rc_h'] = $h;
 return $tn;
}