01.RainbowLetters

<?php
$text = $_GET['text'];
$red = $_GET['red'];
$green = $_GET['green'];
$blue = $_GET['blue'];
$nth = $_GET['nth'];

$hex = "#";
$hex .= str_pad(dechex($red), 2, "0", STR_PAD_LEFT);
$hex .= str_pad(dechex($green), 2, "0", STR_PAD_LEFT);
$hex .= str_pad(dechex($blue), 2, "0", STR_PAD_LEFT);
$hex = strtolower($hex);

$output = "<p>";
for ($i = 0; $i < strlen($text); $i++) {
    if (($i + 1) % $nth == 0) {
        $output .= "<span style=\"color: $hex\">" . htmlspecialchars($text[$i]) . "</span>";
    } else {
        $output .= htmlspecialchars($text[$i]);
    }
}
$output .= "</p>";

echo $output;