Photoshop "Color" blending mode in PHP

<?php

    function Lum($colour) {
        return ($colour['r'] * 0.3) + ($colour['g'] * 0.59) + ($colour['b'] * 0.11);
    }

    function ClipColour($colour) {
        $result     = $colour;
        $luminance  = Lum($colour);

        $cMin = min($colour['r'], $colour['g'], $colour['b']);
        $cMax = max($colour['r'], $colour['g'], $colour['b']);

        if ($cMin < 0.0) {
            $result['r'] = $luminance + ((($colour['r'] - $luminance) * $luminance) / ($luminance - $cMin));
            $result['g'] = $luminance + ((($colour['g'] - $luminance) * $luminance) / ($luminance - $cMin));
            $result['b'] = $luminance + ((($colour['b'] - $luminance) * $luminance) / ($luminance - $cMin));
        }

        if ($cMax > 255) {
            $result['r'] = $luminance + ((($colour['r'] - $luminance) * (255 - $luminance)) / ($cMax - $luminance));
            $result['g'] = $luminance + ((($colour['g'] - $luminance) * (255 - $luminance)) / ($cMax - $luminance));
            $result['b'] = $luminance + ((($colour['b'] - $luminance) * (255 - $luminance)) / ($cMax - $luminance));
        }

        return $result;
    }

    function SetLum($colour, $luminance) {

        $result = array();

        $diff =   $luminance - Lum($colour);

        $result['r'] = $colour['r'] + $diff;
        $result['g'] = $colour['g'] + $diff;
        $result['b'] = $colour['b'] + $diff;

        return ClipColour($result);

    }

    function normalizeColor( $color ) {
        $color['r'] = $color['r'] / 255;
        $color['g'] = $color['g'] / 255;
        $color['b'] = $color['b'] / 255;

        return $color;
    }

    function denormalizeColor( $color ) {
        $color['r'] = round($color['r'] * 255);
        $color['g'] = round($color['g'] * 255);
        $color['b'] = round($color['b'] * 255);

        return $color;
    }

    $overlay_color = array('r'=>180,'g'=>22,'b'=>1, 'a' => 0.35);

    $img = new Imagick();

    if( !isset($_GET['case']) ) {
        $_GET['case'] = '';
    }

    //unmodified version
    $original   = new Imagick('girl.jpg');

    //photoshop image to compare
    $ps = new Imagick('original.jpg');

    $img->addImage($original);
    $it = $original->getPixelIterator();

    foreach( $it as $row => $pixels ) {
        foreach ( $pixels as $column => $pixel ) {
            $rgbIni = $pixel->getColor();

            $rgb = SetLum($overlay_color, Lum($rgbIni));
             $overlay_color     = normalizeColor($overlay_color);
                $rgb        = normalizeColor($rgb);

                $rgbIni         = normalizeColor($rgbIni);

            $rgb['r'] = ((1 - $overlay_color['a']) * $rgbIni['r']) + ($overlay_color['a'] * $rgb['r']);
            $rgb['g'] = ((1 - $overlay_color['a']) * $rgbIni['g']) + ($overlay_color['a'] * $rgb['g']);
            $rgb['b'] = ((1 - $overlay_color['a']) * $rgbIni['b']) + ($overlay_color['a'] * $rgb['b']);

            $test           = denormalizeColor($test);
            $rgb            = denormalizeColor($rgb);
            $overlay_color  = denormalizeColor($overlay_color);

            $pixel->setColor('rgb('.round($rgb['r']).','. round($rgb['g']).','.round($rgb['b']).')');

        }

        $it->syncIterator();
    }

    //add modified version
    $img->addImage($original);
    $img->addImage($ps);

    $img->resetIterator();
    $combined = $img->appendImages(true); //stack images

    header('content-type: image/jpeg');

    $combined->setImageFormat("jpeg");

    echo $combined;

?>