Multi-Color Gradient Fade for Text without CSS?

I'd like help getting this PHP function to work with a third color option.

Note: No CSS. No Javascript. Just PHP and HTML.

<?php

function Gradient($HexFrom, $HexTo, $ColorSteps)
{
        $FromRGB['r'] = hexdec(substr($HexFrom, 0, 2));
        $FromRGB['g'] = hexdec(substr($HexFrom, 2, 2));
        $FromRGB['b'] = hexdec(substr($HexFrom, 4, 2));

        $ToRGB['r'] = hexdec(substr($HexTo, 0, 2));
        $ToRGB['g'] = hexdec(substr($HexTo, 2, 2));
        $ToRGB['b'] = hexdec(substr($HexTo, 4, 2));

        $StepRGB['r'] = ($FromRGB['r'] - $ToRGB['r']) / ($ColorSteps - 1);
        $StepRGB['g'] = ($FromRGB['g'] - $ToRGB['g']) / ($ColorSteps - 1);
        $StepRGB['b'] = ($FromRGB['b'] - $ToRGB['b']) / ($ColorSteps - 1);

        $GradientColors = array();

        for($i = 0; $i <= $ColorSteps; $i++)
        {
                $RGB['r'] = floor($FromRGB['r'] - ($StepRGB['r'] * $i));
                $RGB['g'] = floor($FromRGB['g'] - ($StepRGB['g'] * $i));
                $RGB['b'] = floor($FromRGB['b'] - ($StepRGB['b'] * $i));

                $HexRGB['r'] = sprintf('%02x', ($RGB['r']));
                $HexRGB['g'] = sprintf('%02x', ($RGB['g']));
                $HexRGB['b'] = sprintf('%02x', ($RGB['b']));

                $GradientColors[] = implode(NULL, $HexRGB);
        }
        return $GradientColors;
}

$text = "blah testing blah testing blah";
$length = strlen($text);
$Gradients = Gradient("00FF00", "0000FF", $length);

for ($i=0; $i<$length; $i++) {
    echo '<span style="color: #' . $Gradients[$i] . ';">' . $text[$i] . '</span>';
}


?>

What must I do to get a 3rd option? Such as:

Gradient($HexOne, $HexTwo, $HexThree, $ColorSteps)
                         //^^^^^^^^^