/**
     * Create a thumbnail image from $inputFileName no taller or wider than 
     * $maxSize. Returns the new image resource or false on error.
     * Author: mthorn.net
     */
    function thumbnail($inputFileName, $maxSize = 100)
    {
        $info = getimagesize($inputFileName);

        $type = isset($info['type']) ? $info['type'] : $info[2];

        // Check support of file type
        if ( !(imagetypes() & $type) )
        {
            // Server does not support file type
            return false;
        }

        $width  = isset($info['width'])  ? $info['width']  : $info[0];
        $height = isset($info['height']) ? $info['height'] : $info[1];

        // Calculate aspect ratio
        $wRatio = $maxSize / $width;
        $hRatio = $maxSize / $height;

        // Using imagecreatefromstring will automatically detect the file type
        $sourceImage = imagecreatefromstring(file_get_contents($inputFileName));

        // Calculate a proportional width and height no larger than the max size.
        if ( ($width <= $maxSize) && ($height <= $maxSize) )
        {
            // Input is smaller than thumbnail, do nothing
            return $sourceImage;
        }
        elseif ( ($wRatio * $height) < $maxSize )
        {
            // Image is horizontal
            $tHeight = ceil($wRatio * $height);
            $tWidth  = $maxSize;
        }
        else
        {
            // Image is vertical
            $tWidth  = ceil($hRatio * $width);
            $tHeight = $maxSize;
        }

        $thumb = imagecreatetruecolor($tWidth, $tHeight);

        if ( $sourceImage === false )
        {
            // Could not load image
            return false;
        }

        // Copy resampled makes a smooth thumbnail
        imagecopyresampled($thumb, $sourceImage, 0, 0, 0, 0, $tWidth, $tHeight, $width, $height);
        imagedestroy($sourceImage);

        return $thumb;
    }

    /**
     * Save the image to a file. Type is determined from the extension.
     * $quality is only used for jpegs.
     * Author: mthorn.net
     */
    function imageToFile($im, $fileName, $quality = 80)
    {
        if ( !$im || file_exists($fileName) )
        {
           return false;
        }

        $ext = strtolower(substr($fileName, strrpos($fileName, '.')));

        switch ( $ext )
        {
            case '.gif':
                imagegif($im, $fileName);
                break;
            case '.jpg':
            case '.jpeg':
                imagejpeg($im, $fileName, $quality);
                break;
            case '.png':
                imagepng($im, $fileName);
                break;
            case '.bmp':
                imagewbmp($im, $fileName);
                break;
            default:
                return false;
        }

        return true;
    }

    $im = thumbnail('temp.jpg', 100);
    imageToFile($im, 'temp-thumbnail.jpg');

/**
     * Create a thumbnail image from $inputFileName no taller or wider than 
     * $maxSize. Returns the new image resource or false on error.
     * Author: mthorn.net
     */
    function thumbnail($inputFileName, $maxSize = 100)
    {
        $info = getimagesize($inputFileName);

        $type = isset($info['type']) ? $info['type'] : $info[2];

        // Check support of file type
        if ( !(imagetypes() & $type) )
        {
            // Server does not support file type
            return false;
        }

        $width  = isset($info['width'])  ? $info['width']  : $info[0];
        $height = isset($info['height']) ? $info['height'] : $info[1];;

        // Calculate aspect ratio
        $wRatio = $maxSize / $width;
        $hRatio = $maxSize / $height;

        // Using imagecreatefromstring will automatically detect the file type
        $sourceImage = imagecreatefromstring(file_get_contents($inputFileName));

        // Calculate a proportional width and height no larger than the max size.
        if ( ($width <= $maxSize) && ($height <= $maxSize) )
        {
            // Input is smaller than thumbnail, do nothing
            return $sourceImage;
        }
        elseif ( ($wRatio * $height) < $maxSize )
        {
            // Image is horizontal
            $tHeight = ceil($wRatio * $height);
            $tWidth  = $maxSize;
        }
        else
        {
            // Image is vertical
            $tWidth  = ceil($hRatio * $width);
            $tHeight = $maxSize;
        }

        $thumb = imagecreatetruecolor($tWidth, $tHeight);

        if ( $sourceImage === false )
        {
            // Could not load image
            return false;
        }

        // Copy resampled makes a smooth thumbnail
        imagecopyresampled($thumb, $sourceImage, 0, 0, 0, 0, $tWidth, $tHeight, $width, $height);
        imagedestroy($sourceImage);

        return $thumb;
    }

    /**
     * Save the image to a file. Type is determined from the extension.
     * $quality is only used for jpegs.
     * Author: mthorn.net
     */
    function imageToFile($im, $fileName, $quality = 80)
    {
        if ( !$im || file_exists($fileName) )
        {
           return false;
        }

        $ext = strtolower(substr($fileName, strrpos($fileName, '.')));

        switch ( $ext )
        {
            case '.gif':
                imagegif($im, $fileName);
                break;
            case '.jpg':
            case '.jpeg':
                imagejpeg($im, $fileName, $quality);
                break;
            case '.png':
                imagepng($im, $fileName);
                break;
            case '.bmp':
                imagewbmp($im, $fileName);
                break;
            default:
                return false;
        }

        return true;
    }

    $im = thumbnail('temp.jpg', 100);
    imageToFile($im, 'temp-thumbnail.jpg');

<?php 
    /**
     * Create a thumbnail image from $inputFileName no taller or wider than 
     * $maxSize. Returns the new image resource or false on error.
     * Author: mthorn.net
     */
    function thumbnail($inputFileName, $maxSize = 100)
    {
        $info = getimagesize($inputFileName);

        $type = isset($info['type']) ? $info['type'] : $info[2];

        // Check support of file type
        if ( !(imagetypes() & $type) )
        {
            // Server does not support file type
            return false;
        }

        $width  = isset($info['width'])  ? $info['width']  : $info[0];
        $height = isset($info['height']) ? $info['height'] : $info[1];;

        // Calculate aspect ratio
        $wRatio = $maxSize / $width;
        $hRatio = $maxSize / $height;

        // Using imagecreatefromstring will automatically detect the file type
        $sourceImage = imagecreatefromstring(file_get_contents($inputFileName));

        // Calculate a proportional width and height no larger than the max size.
        if ( ($width <= $maxSize) && ($height <= $maxSize) )
        {
            // Input is smaller than thumbnail, do nothing
            return $sourceImage;
        }
        elseif ( ($wRatio * $height) < $maxSize )
        {
            // Image is horizontal
            $tHeight = ceil($wRatio * $height);
            $tWidth  = $maxSize;
        }
        else
        {
            // Image is vertical
            $tWidth  = ceil($hRatio * $width);
            $tHeight = $maxSize;
        }

        $thumb = imagecreatetruecolor($tWidth, $tHeight);

        if ( $sourceImage === false )
        {
            // Could not load image
            return false;
        }

        // Copy resampled makes a smooth thumbnail
        imagecopyresampled($thumb, $sourceImage, 0, 0, 0, 0, $tWidth, $tHeight, $width, $height);
        imagedestroy($sourceImage);

        return $thumb;
    }

    /**
     * Save the image to a file. Type is determined from the extension.
     * $quality is only used for jpegs.
     * Author: mthorn.net
     */
    function imageToFile($im, $fileName, $quality = 80)
    {
        if ( !$im || file_exists($fileName) )
        {
           return false;
        }

        $ext = strtolower(substr($fileName, strrpos($fileName, '.')));

        switch ( $ext )
        {
            case '.gif':
                imagegif($im, $fileName);
                break;
            case '.jpg':
            case '.jpeg':
                imagejpeg($im, $fileName, $quality);
                break;
            case '.png':
                imagepng($im, $fileName);
                break;
            case '.bmp':
                imagewbmp($im, $fileName);
                break;
            default:
                return false;
        }

        return true;
    }

    $im = thumbnail('temp.jpg', 100);
    imageToFile($im, 'temp-thumbnail.jpg');
?>

http://www.mthorn.net

Will create a thumnail no taller or wider than the supplied size. Also contains some tips for reading and writing images using GD.

Create a thumbnail maintaining aspect ratio using GD

php

PHP