calculate image (re)size and keep ratio

ImgTools.php

<?php

class ImgTools
{
    /**
     * Calculate image dimensions from given ratio and desired new x-/y- values
     *
     * @param int  $desired_width          desired image width
     * @param int  $desired_height         desired image height
     * @param int  $ratio                  desired ratio
     * @param bool $new_values_are_maximum true if the resulting image should not be larger in any dimension (max size), 
     *                                     false if the image should have at least the given size (min size)
     * @return array containing new x and y values matching the ratio
     * @throws \RuntimeException
     */
    public static function calculateImageSize($desired_width, $desired_height, $ratio, $new_values_are_maximum = true)
    {
        if (empty($ratio))
        {
            throw new \RuntimeException('Image ratio cannot be 0.');
        }

        $width = $desired_width;
        $height = $desired_height;

        if (empty($width))
        {
            $width = $height * $ratio;
        }
        elseif (empty($height))
        {
            $height = $width / $ratio;
        }

        if ($new_values_are_maximum) $width = $height * $ratio;
        else $height = $width / $ratio;

        if ($height > $desired_height == $new_values_are_maximum)
        {
            $width = $width / $height * $desired_height;
            $height = $desired_height;
        }
        elseif ($width > $desired_width == $new_values_are_maximum)
        {
            $height = $height / $width * $desired_width;
            $width = $desired_width;
        }
        return [$width, $height];
    }
}