This is an old function I wrote a bazillion years ago for calculating the dimensions of an image (aspect-ratio-preserved) that would fit within a fixed-size box (specified by $max_width, $max_height). Still useful.

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/*----------------------------------------------------------------------------*\
  Function:    get_thumb_dimensions
  Description: examines an image and calculates thumbnail dimensions for it, based
                   on the max width and height dimensions passed in as parameters.
                   Returns an array containing the width [0] and height [1]
  Parameters: $image      - the image name and path
                   $max_width  - the max width of the thumbnail
                   $max_height - the max height of the thumbnail
\*----------------------------------------------------------------------------*/
function get_thumb_dimensions($image_path, $max_width, $max_height)
{
  // find the images dimensions
  $image_dimensions = getImageSize($image_path);
  $image_width      = $image_dimensions[0];
  $image_height     = $image_dimensions[1];

  // default the thumbnail dimensions to the maximum values
  $thumb_height = $max_height;
  $thumb_width  = $max_width;

  // assume image width >= height
  $scaling_ratio = $image_width / $max_width;

  // compare the scaled height with the maximum thumb height. If it's bigger,
  // we need to find a new scaling ratio
  if (($image_height / $scaling_ratio)  > $max_height)
  {
    $scaling_ratio = $image_height / $max_height;
    $thumb_width = $image_width / (float) $scaling_ratio;
  }
  else
    $thumb_height = $image_height / (float) $scaling_ratio;

  // now return the dimensions
  $thumb_dimensions = array($thumb_width, $thumb_height);

  return $thumb_dimensions;
}