怎么样使用PHP实现图片按比例截取功能？

filename = 'img/test.jpg';
$all_type = array(
"jpg" => array("create"=>"ImageCreateFromjpeg", "output"=>"imagejpeg" , "exn"=>".jpg"),
"gif" => array("create"=>"ImageCreateFromGIF" , "output"=>"imagegif" , "exn"=>".gif"),
"jpeg" => array("create"=>"ImageCreateFromjpeg", "output"=>"imagejpeg" , "exn"=>".jpg"),
"png" => array("create"=>"imagecreatefrompng" , "output"=>"imagepng" , "exn"=>".png"),
"wbmp" => array("create"=>"imagecreatefromwbmp", "output"=>"image2wbmp" , "exn"=>".wbmp")
);
$imgtype = getimagesize($filename);
$width = $imgtype[0];
$height = $imgtype[1];
$type = str_replace('image/','',$imgtype['mime']);
$func_create = $all_type[$type]['create'];
$func_output = $all_type[$type]['output'];
$x = $y =0;
if(($width * 100)>($height * 120))
{
$newwidth = ceil($height * 120/100);
$newheight = $height;
$x = ($width-$newwidth)/2;
}
elseif(($width * 100)<($height * 120))
{
$newheight = ceil($width * 100/120);
$newwidth = $width;
$y = ($height-$newheight)/2;
}
else
{
$newheight = $height;
$newwidth = $width; 
}
// Load
$thumb = imagecreatetruecolor($newwidth, $newheight);
$source = $func_create($filename);
// Resize
imagecopyresized($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $newwidth, $newheight);
// Output
$func_output($thumb,'a.jpeg');