helper.php

<?php
/**
 * @package    J51_ImageHover
 * @copyright  Copyright (C) 2009 - 2017 Joomla51. All rights reserved.
 * @license    GPL v3.0 or later http://www.gnu.org/licenses/gpl-3.0.html
 */

// no direct access
defined('_JEXEC') or die('Restricted access');

class modJ51ImageHover
{
	public function adjustBrightness($hex, $steps)
	{
		// Steps should be between -255 and 255. Negative = darker, positive = lighter
		$steps = max(-255, min(255, $steps));

		// Normalize into a six character long hex string
		$hex = str_replace('#', '', $hex);
		if (strlen($hex) == 3)
		{
			$hex = str_repeat(substr($hex, 0, 1), 2) . str_repeat(substr($hex, 1, 1), 2) . str_repeat(substr($hex, 2, 1), 2);
		}

		// Split into three parts: R, G and B
		$color_parts = str_split($hex, 2);
		$return = '#';

		foreach ($color_parts as $color)
		{
			$color   = hexdec($color); // Convert to decimal
			$color   = max(0, min(255, $color + $steps)); // Adjust color
			$return .= str_pad(dechex($color), 2, '0', STR_PAD_LEFT); // Make two char hex code
		}

		return $return;
	}
}