Sort colors from darkest to lightest

sort-colors.php

<?php
/**
 * Sort colors from dark to light
 * http://stackoverflow.com/questions/20213421/sorting-color-selections-on-brightness
 */
private static function sortColors($colors) {
    usort(
        $colors,
        function ($one, $two) {
            return self::colorToLum($one) - self::colorToLum($two);
        }
    );

    return $colors;
}

private static function colorToLum($color) {
    $red = hexdec(substr($color, 1, 2));
    $green = hexdec(substr($color, 3, 2));
    $blue = hexdec(substr($color, 5, 2));

    return (0.299 * $red + 0.587 * $green + 0.114 * $blue);
}