<?php

function getColor($num) {
    $hash = md5('color' . $num);
    return array(hexdec(substr($hash, 0, 2)), hexdec(substr($hash, 2, 2)), hexdec(substr($hash, 4, 2)));
}

$nums = range(0, 20);
$scale = 30;
$im = imagecreatetruecolor($scale * count($nums), $scale);
$i = 0;
foreach ($nums as $num) {
    list($r,$g,$b) = getColor($num);
    $c = imagecolorallocate($im, $r, $g, $b);
    imagefilledrectangle($im, $scale * $i, 0, $scale * ($i + 1), $scale, $c);
    $i++;
}

header('Content-Type: image/png');
imagepng($im);