How to properly resize large images in PHP

For a website I run, users can upload pictures they've drawn to a gallery. We create a thumbnail and a small view of that image to display to other users (clicking the small view image shows the full sized image).

With this in mind, I created a very simple resize script. In most cases this script works perfectly. However, I've come across a single odd case in which the script messes up entirely.

When running the file http://img191.imageshack.us/img191/2268/935full.png (1641x3121) through the script (which creates one thumbnail with a max width or height of 150 and another of 400) we get a perfect thumbnail http://img267.imageshack.us/img267/5803/935thumb.png (78x150) and a small view image sized properly, but which is cut off and stretched http://img28.imageshack.us/img28/4002/935show.png (211 x 400).

With that in mind, my question is: Is this a problem in PHP or a logic error? And how can I fix it?

Thank you for your time. The code I use to create these thumbnails is below.

<?php
/**
 * Creates a thumbnail for any type of pre-existing image. Always saves as PNG image
 *
 * @param string - The location of the pre-existing image.
 * @param string - The location to save the thumbnail, including filename and extension.
 * @param int    - The Maximum Width, Default of 150
 * @param int    - The Maximum Height, Default of 150
 * @return bool  - Success of saving the thumbnail.
 */
function imagecreatethumbnail($file,$output,$max_width = 150,$max_height = 150)
{
        $img = imagecreatefromstring(file_get_contents($file));
        list($width, $height, $type, $attr) = getimagesize($file);
        if($height > $max_height || $width > $max_width)
        {
                if($width > $height)
                {
                        $thumb_width = $max_width;
                        $thumb_height = ceil(($height * $thumb_width)/$width);
                }
                else
                {
                        $thumb_height = $max_height;
                        $thumb_width = ceil(($width * $thumb_height)/$height);
                }
        } else {
                $thumb_width = $width;
                $thumb_height = $height;
        }
        imagesavealpha($img,true);
        $thumb = imagecreatetruecolor(开发者_如何学C$thumb_width,$thumb_height);
        imagesavealpha($thumb,true);
        imagealphablending($thumb,false);
        imagecopyresampled($thumb,$img,0,0,0,0,$thumb_width,$thumb_height,$width,$height);
        $return = imagepng($thumb,$output);
        imagedestroy($img);
        imagedestroy($thumb);
        return $return;
}

Try this library and tell me if the same occurs:

http://phpthumb.gxdlabs.com/

Have you tried removing the ceil() functions? The image functions will automatically convert floats to integers anyway.

EDIT:

See if this works:

if($width > $max_width && $height * $thumb_width / $width < $max_width)
{
    $thumb_width = $max_width;
    $thumb_height = $height * $thumb_width / $width;
}
elseif($height > $max_height && $width * $thumb_height / $height < $max_height)
{
    $thumb_height = $max_height;
    $thumb_width = $width * $thumb_height / $height;
}
else
{
    $thumb_width = $width;
    $thumb_height = $height;
}

This seems to be a bug in PHP.