php image luminance detector

function get_avg_luminance($filename, $num_samples=10) {
    $img = imagecreatefromjpeg($filename);

    $width = imagesx($img);
    $height = imagesy($img);

    $x_step = intval($width/$num_samples);
    $y_step = intval($height/$num_samples);

    $total_lum = 0;

    $sample_no = 1;

    for ($x=0; $x<$width; $x+=$x_step) {
        for ($y=0; $y<$height; $y+=$y_step) {

            $rgb = imagecolorat($img, $x, $y);
            $r = ($rgb >> 16) & 0xFF;
            $g = ($rgb >> 8) & 0xFF;
            $b = $rgb & 0xFF;

            // choose a simple luminance formula from here
            // http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
            $lum = ($r+$r+$b+$g+$g+$g)/6;

            $total_lum += $lum;

            $sample_no++;
        }
    }

    // work out the average
    $avg_lum  = $total_lum/$sample_no;
    return $avg_lum;
    // assume a medium gray is the threshold, #acacac or RGB(172, 172, 172)
    // this equates to a luminance of 170
}