PHP调整图像尺寸

    function resize_image($filename, $tmpname, $xmax, $ymax) {

         $ext = explode(".", $filename);

         $ext = $ext[count($ext)-1];

         if($ext == "jpg" || $ext == "jpeg") $im = imagecreatefromjpeg($tmpname);

         elseif($ext == "png") $im = imagecreatefrompng($tmpname);

         elseif($ext == "gif") $im = imagecreatefromgif($tmpname);

         $x = imagesx($im);

         $y = imagesy($im);

         if($x<=$xmax && $y <=$ymax) return $im;

         if($x>= $y) {

             $newx = $xmax;

             $newy = $newx * $y / $x;

         } else {

             $newy = $ymax;

             $newx = $x / $y * $newy;

         }

         $im2 = imagecreatetruecolor($newx, $newy);

         imagecopyresized($im2, $im, 0, 0, 0, 0, floor($newx), floor($newy), $x, $y);

         return $im2;

    }