This uses the GD library to create a thumbnail out of an image.

$infile = path to source image

$outfile = file and path to write the thumbnail to

$maxw = Maximum width of the thumbnail

$maxh = Maximum height of the thumbnail

$stretch = Stretch image if it is smaller than the maxw and maxh
The function will preserve the aspect ratio of the image. If you specify 0 for either $maxw or $maxh then the function will assume that the max width/height can be unlimited (ie, if you specify 0 for $maxw it will only resize the height to $maxh likewise for specifying 0 for $maxh)



By default if the source image is already smaller than the thumbnail size it will not be stretched to the $maxw or $maxh. If you specify TRUE for $stretch it will be stretched if it is already smaller than the $maxw and $maxh parameters.



This function supports PNG, JPEG and GIF formats. Adding new formats shouldn't be hard.

<?php
/*
  Tye Shavik (tye at tye . ca)
*/
function create_thumbnail($infile,$outfile,$maxw,$maxh,$stretch = FALSE) {
  clearstatcache();
  if (!is_file($infile)) {
    trigger_error("Cannot open file: $infile",E_USER_WARNING);
    return FALSE;
  }
  if (is_file($outfile)) {
      trigger_error("Output file already exists: $outfile",E_USER_WARNING);
    return FALSE;
  }

  $functions = array(
    'image/png' => 'ImageCreateFromPng',
    'image/jpeg' => 'ImageCreateFromJpeg',
  );

  // Add GIF support if GD was compiled with it
  if (function_exists('ImageCreateFromGif')) { $functions['image/gif'] = 'ImageCreateFromGif'; }

  $size = getimagesize($infile);

  // Check if mime type is listed above
  if (!$function = $functions[$size['mime']]) {
      trigger_error("MIME Type unsupported: {$size['mime']}",E_USER_WARNING);
    return FALSE;
  }

  // Open source image
  if (!$source_img = $function($infile)) {
      trigger_error("Unable to open source file: $infile",E_USER_WARNING);
    return FALSE;
  }

  $save_function = "image" . strtolower(substr(strrchr($size['mime'],'/'),1));

  // Scale dimensions
  list($neww,$newh) = scale_dimensions($size[0],$size[1],$maxw,$maxh,$stretch);

  if ($size['mime'] == 'image/png') {
    // Check if this PNG image is indexed
    $temp_img = imagecreatefrompng($infile);
    if (imagecolorstotal($temp_img) != 0) {
      // This is an indexed PNG
      $indexed_png = TRUE;
    } else {
      $indexed_png = FALSE;
    }
    imagedestroy($temp_img);
  }

  // Create new image resource
  if ($size['mime'] == 'image/gif' || ($size['mime'] == 'image/png' && $indexed_png)) {
    // Create indexed 
    $new_img = imagecreate($neww,$newh);
    // Copy the palette
    imagepalettecopy($new_img,$source_img);

    $color_transparent = imagecolortransparent($source_img);
    if ($color_transparent >= 0) {
      // Copy transparency
      imagefill($new_img,0,0,$color_transparent);
      imagecolortransparent($new_img, $color_transparent);
    }
  } else {
    $new_img = imagecreatetruecolor($neww,$newh);
  }

  // Copy and resize image
  imagecopyresampled($new_img,$source_img,0,0,0,0,$neww,$newh,$size[0],$size[1]);

  // Save output file
  if ($save_function == 'imagejpeg') {
      // Change the JPEG quality here
      if (!$save_function($new_img,$outfile,75)) {
          trigger_error("Unable to save output image",E_USER_WARNING);
          return FALSE;
      }
  } else {
      if (!$save_function($new_img,$outfile)) {
          trigger_error("Unable to save output image",E_USER_WARNING);
          return FALSE;
      }
  }

  // Cleanup
  imagedestroy($source_img);
  imagedestroy($new_img);

  return TRUE;
}
// Scales dimensions
function scale_dimensions($w,$h,$maxw,$maxh,$stretch = FALSE) {
    if (!$maxw && $maxh) {
      // Width is unlimited, scale by width
      $newh = $maxh;
      if ($h < $maxh && !$stretch) { $newh = $h; }
      else { $newh = $maxh; }
      $neww = ($w * $newh / $h);
    } elseif (!$maxh && $maxw) {
      // Scale by height
      if ($w < $maxw && !$stretch) { $neww = $w; }
      else { $neww = $maxw; }
      $newh = ($h * $neww / $w);
    } elseif (!$maxw && !$maxh) {
      return array($w,$h);
    } else {
      if ($w / $maxw > $h / $maxh) {
        // Scale by height
        if ($w < $maxw && !$stretch) { $neww = $w; }
        else { $neww = $maxw; }
        $newh = ($h * $neww / $w);
      } elseif ($w / $maxw <= $h / $maxh) {
        // Scale by width
        if ($h < $maxh && !$stretch) { $newh = $h; }
        else { $newh = $maxh; }
        $neww = ($w * $newh / $h);
      }
    }
    return array(round($neww),round($newh));
}
?>