<?php

if(isset($_GET["text"]) && isset($_GET["red"]) && isset($_GET["green"]) &&
    isset($_GET["blue"]) && isset($_GET["nth"])){

    $text =  $_GET["text"];
    $red = $_GET["red"];
    $green = $_GET["green"];
    $blue = $_GET["blue"];
    $nth = $_GET["nth"];
}


function fromRGB($red, $green, $blue){

    $red=dechex($red);
    if(strlen($red)<2)
        $red='0'.$red;

    $green=dechex($green);
    if(strlen($green)<2)
        $green='0'.$green;

    $blue=dechex($blue);
    if(strlen($blue)<2)
        $blue='0'.$blue;

    return '#' . $red . $green . $blue;

}


$hex = fromRGB($red, $green, $blue);

$openParagraph = "<p>";
$closeParagraph = "</p>";

$openSpanStart = "<span style=\"color: ";
$openSpanEnd = "\">";
$closeSpan = "</span>";

echo $openParagraph;

$stringLength = strlen($text);
for($i = 0; $i < $stringLength; $i++) {

    $char = substr($text, $i, 1);
    $char = htmlspecialchars($char);
    if( (($i + 1) % $nth === 0)){

        echo($openSpanStart.$hex.$openSpanEnd.$char.$closeSpan);
    }
    else{
        echo $char;
    }

}
echo $closeParagraph;

<p></p>