创建图像缩略图需要许多时间，此代码将有助于了解缩略图的逻辑。

 
/********************** 
*@filename - path to the image 
*@tmpname - temporary path to thumbnail 
*@xmax - max width 
*@ymax - max height 
*/ 
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; 
} 

这里是摘自脚本之家之前发布的文章。更多的技巧可以参考。
收集的二十一个实用便利的PHP函数代码