<?php

/**
 * Change the brightness of the passed in color
 *
 * $diff should be negative to go darker, positive to go lighter and
 * is subtracted from the decimal (0-255) value of the color
 *
 * @param string $hex color to be modified
 * @param string $diff amount to change the color
 * @return string hex color
 */
public function hex_color_mod($hex, $diff) {
	$rgb = str_split(trim($hex, '# '), 2);

	foreach ($rgb as &$hex) {
		$dec = hexdec($hex);
		if ($diff >= 0) {
			$dec += $diff;
		}
		else {
			$dec -= abs($diff);
		}
		$dec = max(0, min(255, $dec));
		$hex = str_pad(dechex($dec), 2, '0', STR_PAD_LEFT);
	}

	return '#'.implode($rgb);
}