Shift Hex Color

<?php
/**
 * Color shift a hex value by a specific percentage factor
 *
 * @param string $supplied_hex Any valid hex value. Short forms e.g. #333 accepted.
 * @param string $shift_method How to shift the value e.g( +,up,lighter,>) 
 * @param integer $percentage Percentage in range of [0-100] to shift provided hex value by
 * @return string shifted hex value
 * @version 1.0 2008-03-28
 */
 
function hex_shift($supplied_hex,$shift_method,$percentage=50)
{
	$shifted_hex_value     = null;
	$valid_shift_option    = FALSE;
	$current_set           = 1;
	$RGB_values            = array();
	$valid_shift_up_args   = array('up','+','lighter','>');
	$valid_shift_down_args = array('down','-','darker','<');
	$shift_method          = strtolower(trim($shift_method));
 
	// Check Factor
	if(!is_numeric($percentage)||($percentage = (int) $percentage)<0||$percentage>100) {
		trigger_error( "Invalid factor", E_USER_ERROR );
	}
 
	// Check shift method
	foreach(array($valid_shift_down_args,$valid_shift_up_args) as $options){
		foreach($options as $method) {
			if($method == $shift_method) {
				$valid_shift_option = !$valid_shift_option;
				$shift_method = ($current_set === 1) ? '+' : '-';				
				break 2;
			}
		}
		++$current_set;
	}
 
	if(!$valid_shift_option) {
		trigger_error( "Invalid shift method", E_USER_ERROR );
	}
 
	// Check Hex string
	switch(strlen($supplied_hex=(str_replace('#','',trim($supplied_hex))))) {
		case 3:
			if(preg_match('/^([0-9a-f])([0-9a-f])([0-9a-f])/i',$supplied_hex)) {
				$supplied_hex = preg_replace('/^([0-9a-f])([0-9a-f])([0-9a-f])/i',
				'\\1\\1\\2\\2\\3\\3',$supplied_hex);
			} else {
				trigger_error( "Invalid hex value", E_USER_ERROR );
			}
			break;
		case 6:
			if(!preg_match('/^[0-9a-f]{2}[0-9a-f]{2}[0-9a-f]{2}$/i',$supplied_hex)) {
				trigger_error( "Invalid hex value", E_USER_ERROR );
			}
			break;
		default: 
			trigger_error( "Invalid hex length", E_USER_ERROR );
	}
 
	// Start shifting
	$RGB_values['R'] = hexdec($supplied_hex{0}.$supplied_hex{1});
	$RGB_values['G'] = hexdec($supplied_hex{2}.$supplied_hex{3});
	$RGB_values['B'] = hexdec($supplied_hex{4}.$supplied_hex{5});
 
	foreach($RGB_values as $c => $v) {
		switch($shift_method) {
			case '-':
				$amount = round(((255-$v)/100)*$percentage)+$v;
			break;
			case '+':
				$amount = $v-round(($v/100)*$percentage);
			break;
			default:
				trigger_error( "Oops. Unexpected shift method", E_USER_ERROR );
		}
 
		$shifted_hex_value .= $current_value = (
			strlen($decimal_to_hex = dechex($amount)) < 2
			) ? '0'.$decimal_to_hex : $decimal_to_hex;
	}
 
	return '#'.$shifted_hex_value;	
}
 
// ---------------------------------------------------
// Example. Black to white fade by an increment of 10%
// ---------------------------------------------------
for ($i=0; $i <= 100; $i+=10) { 
	echo hex_shift('#000','up',$i),"\n";
}
 
?>