<?php

function html2rgb($color) 
{
	$color = ltrim($color, '#');

	$r = hexdec(substr($color, 0, 2)); 
	$g = hexdec(substr($color, 2, 2)); 
	$b = hexdec(substr($color, 4, 2)); 
	return array($r, $g, $b);
}

function diff360($a, $b)
{
	if ($a > $b) list($a, $b) = array($b, $a);
	$c = $b-$a;
	return ($c < 180) ? $c : 360+($a-$b);
}

function rgb2hls($rgb)
{
	for ($c=0; $c<3; $c++) {
		$rgb[$c] = $rgb[$c] / 255;
	}

	$hls = array(0, 0, 0);
	$max = max($rgb);
	$min = min($rgb);

	$hls[1] = ($max + $min) / 2;
	if ($max == $min) {
		$hls[0] = 0;
		$hls[2] = 0;
	} else {
		$delta = $max - $min;
		$hls[2] = ($hls[1] <= 0.5) ? ($delta / ($max + $min)) : ($delta / (2 - $delta));

		if ($rgb[0] == $max) {
			$hls[0] = ($rgb[1] - $rgb[2]) / $delta;
		} else if ($rgb[1] == $max) {
			$hls[0] = 2 + ($rgb[2] - $rgb[0]) / $delta;
		} else {
			$hls[0] = 4 + ($rgb[0] - $rgb[1]) / $delta;
		}

		$hls[0] *= 60;
		if ($hls[0] < 0) {
			$hls[0] += 360;
		}
		if ($hls[0] > 360) {
			$hls[0] -= 360;
		}
	}
	ksort($hls);
	return $hls;
}

function similar_color($rgb1, $rgb2, $tolerance = 10)
{
	$hls1 = rgb2hls($rgb1);
	$hls2 = rgb2hls($rgb2);
	return (diff360($hls1[0], $hls2[0]) / 1.8) < $tolerance;
}

function test_similar_color($test, $matches)
{
	$span = '<span style="color:%s;">%s</span>';
	$c1 = sprintf($span, $test, $test);
	$rgb = html2rgb($test);
	
	foreach($matches as $color)
	{
		$c2 = sprintf($span, $color, $color);
		echo "$c1 similaire à $c2 ? ".(similar_color($rgb, html2rgb($color)) ? 'Oui !' : 'Non...')."<br />\n";
	}
}

test_similar_color('#ff0000', array('#ff3333', '#ff6666', '#cc0000', '#cc3333'));

function html2rgb($color)
{
  // gestion du #...
  if (substr($color,0,1)=="#") $color=substr($color,1,6);

  $tablo[0] = hexdec(substr($color, 0, 2));
  $tablo[1] = hexdec(substr($color, 2, 2));
  $tablo[2] = hexdec(substr($color, 4, 2));
  return $tablo;
}