php如何实现调整图片大小或创建缩略图

<?php
// 重置图片文件大小
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; 
 }
 ?>