Resize and thumbnail with fixed width and variable height

I have a script that uploads an image but first it resizes the image before upload and again to create a thumbnail.
Right now it has a fixed landscape width and height but I realized that some of my uploads will have a longer height. How can I modify my code to make the width fixed but the height will adjust regardless of a landscape or portrait orientation. Here is the parts of the code:

function makeThumbnails($updir, $img){
      $thumbnail_width = 560;
      $thumbnail_height = 432;   ----------------------> This needs to change regardless of orientation
      $new_image = imagecreatetruecolor($thumbnail_width, $thumbnail_height);
    imagecopyresampled($new_image, $old_image, $dest_x, $dest_y, 0, 0, $new_width, 
     $new_height, $original_width, $original_height);

This is some old code of mine based on a maximum width but you should get the idea.:

// Temporary upload image name
$original_image = '../original_images/flowers.jpg';
// Get the image dimensions
$size=GetImageSize( $original_image );
// Maximum image width
$max_width = '100';
// Maximum image height
//$max_height = '100';
$ratio = 100/$size[0];
$max_height = $size[1]*$ratio;
// Resize the image and save		
$src_img = ImageCreateFromJPEG( $original_image );
$thumbnail = ImageCreateTrueColor( $max_width, $max_height );
ImageCopyResampled( $thumbnail, $src_img, 0, 0, 0, 0, $max_width, $max_height, $size[0],$size[1] );
ImageJPEG( $thumbnail, 'flowers_GD.jpg' );
ImageDestroy( $thumbnail );

Or you could use imagemagick:

exec("convert input -resize 560x output");

You could also use Imagick although I do not use it so you would have to search for the code.

Thank you! Determining the ratio should work.