PHP Exam - Rainbow Letters

<?php

$text = trim($_GET['text']);
$red = dechex((int) trim($_GET['red']));
$green = dechex((int) trim($_GET['green']));
$blue = dechex((int) trim($_GET['blue']));
$n = (int) trim($_GET['nth']);

$color = str_pad($red, 2, '0', STR_PAD_LEFT) . str_pad($green, 2, '0', STR_PAD_LEFT) . str_pad($blue, 2, '0', STR_PAD_LEFT);

$result = '<p>';

for ($i = 0; $i < strlen($text); $i++) {
    if (($i + 1) % $n == 0) {
        $result .= '<span style="color: #' . strtolower($color) . '">' . htmlspecialchars($text[$i]) . '</span>';
    } else {
        $result .= htmlspecialchars($text[$i]);
    }
}

$result .= '</p>';

echo $result;