Lighten or darken a given colour

color_luminance.php

<?php
/**
 * Lightens/darkens a given colour (hex format), returning the altered colour in hex format.7
 * @param str $hex Colour as hexadecimal (with or without hash);
 * @percent float $percent Decimal ( 0.2 = lighten by 20%(), -0.4 = darken by 40%() )
 * @return str Lightened/Darkend colour as hexadecimal (with hash);
 */
function color_luminance( $hex, $percent ) {
	
	// validate hex string
	
	$hex = preg_replace( '/[^0-9a-f]/i', '', $hex );
	$new_hex = '#';
	
	if ( strlen( $hex ) < 6 ) {
		$hex = $hex[0] + $hex[0] + $hex[1] + $hex[1] + $hex[2] + $hex[2];
	}
	
	// convert to decimal and change luminosity
	for ($i = 0; $i < 3; $i++) {
		$dec = hexdec( substr( $hex, $i*2, 2 ) );
		$dec = min( max( 0, $dec + $dec * $percent ), 255 ); 
		$new_hex .= str_pad( dechex( $dec ) , 2, 0, STR_PAD_LEFT );
	}		
	
	return $new_hex;
}