Rainbow Letters

<?php
$text = $_GET['text'];
$letterNumber = (int)$_GET['nth'];
$colorNum = array((int)$_GET['red'], (int)$_GET['green'], (int)$_GET['blue']);
$color = '#';
foreach ($colorNum as $num) {
    $hex = dechex($num);
    if (strlen($hex) === 1) {
        $hex = '0' . $hex;
    }
    $color .= $hex;
}

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

echo $result . '</p>';