Auto Level Image using PHP GD

function auto_level_image($image, $level = 95, $dynamic = 254) {
    /* Adjusts levels on image by cutting out insignificant colors.
     *
     * $level is 1-100, defining what % of the range of colors you
     * want to keep in the final image.
     * This is a modified function.  Check out the original at,
     * http://www.giacobbe85.altervista.org/down_en/info/Software_and_scripts/Automatic_color_correction_in_images.php?lang=eng
     */

    $colors = array('red', 'green', 'blue');
    $pixels = 0;

    // Create arrays to keep track of color frequency
    $empty_array = array();
    $red_freq = array_pad($empty_array, 256, 0);
    $green_freq = array_pad($empty_array, 256, 0);
    $blue_freq = array_pad($empty_array, 256, 0);

    // Scans each image pixel
    for ($x=0; $x < imagesx($image); $x++) {

        for($y=0; $y < imagesy($image); $y++) {

            // Computes RGB for the current pixel
            $pixel_color = imagecolorat($image, $x, $y);
            $red = ($pixel_color >> 16) & 255;
            $green = ($pixel_color >> 8) & 255;
            $blue = $pixel_color & 255;
            $pixels += 1;

            // Adds colors to frequency arrays
            $red_freq[$red] += 1;
            $green_freq[$green] += 1;
            $blue_freq[$blue] += 1;

            // Searches for the minimum
            if ($x == 0 && $y == 0) {
                $red_min = $red;
                $green_min = $green;
                $blue_min = $blue;
            } else {
                if($red < $red_min) $red_min = $red;
                if($green < $green_min) $green_min = $green;
                if($blue < $blue_min) $blue_min = $blue;
            }

            // Searches for the maximum
            if($red > $red_max) $red_max = $red;
            if($green > $green_max) $green_max = $green;
            if($blue > $blue_max) $blue_max = $blue;
        }
    }

    // Find range of acceptable colors from $level
    $range = $pixels * ($level / 100);
    $outliers = ($pixels - $range) / 2;

    foreach ($colors as $color) {
        ${$color . "_outliers_min"} = $outliers;
        ${$color . "_outliers_max"} = $outliers + $range;
        $stop_min = false;

        foreach (${$color . "_freq"} as $key => ${$color . "_lvl"}) {
            ${$color . "_outliers_min"} -= ${$color . "_lvl"};
            ${$color . "_outliers_max"} -= ${$color . "_lvl"};

            if (${$color . "_outliers_min"} <= 0 && $stop_min != true) {
                ${$color . "_min"} = $key;
                $stop_min = true;
            }

            if (${$color . "_outliers_max"} <= 0) {
                ${$color . "_max"} = $key;
                break;
            }
        }
    }

    # Normalization variables
    foreach ($colors as $color) {
        ${"shift_" . $color} = ${$color . "_min"};

        if (${$color . "_max"} != ${"shift_" . $color}) {
            ${"scale_" . $color} = $dynamic / (${$color . "_max"} - ${"shift_" . $color});
        } else {
            ${"scale_" . $color} = 1;
        }
    }

    // Now to edit the image with all our new numbers!
    // -----------------------------------------------

    // Scans each pixel to normalize it
    for ($x=0; $x < imagesx($image); $x++) {

        for ($y=0; $y < imagesy($image); $y++) {

            // Computes the RGB values for the current pixel
            $pixel_color = imagecolorat($image, $x, $y);
            $red = ($pixel_color >> 16) & 255;
            $green = ($pixel_color >> 8) & 255;
            $blue = $pixel_color & 255;

            foreach ($colors as $color) {

                // get new color numbers
                ${"new_" . $color} = ($$color - ${"shift_" . $color}) * ${"scale_" . $color};

                // keep colors in range of min and max
                if (${"new_" . $color} > 255) {
                    ${"new_" . $color} = 255;
                } elseif (${"new_" . $color} < 0) {
                    ${"new_" . $color} = 0;
                }
            }

            // Computes the normalized pixel and saves it in the image
            $color_pixel = imagecolorallocate($image, $new_red, $new_green, $new_blue);
            imagesetpixel($image, $x, $y, $color_pixel);
        }
    }

    return 1;
}