Create a color palette from an image.

create-palette.php

<?php

$img = imagecreatefrompng ( '/tmp/image.png' );
$palette = build_palette($img, 20, 20);

$unique_colors = array();
foreach($palette as $row) {
    foreach($row as $col) {
        $rgb = sprintf('%02x%02x%02x', $col['r'], $col['g'], $col['b']);
        if (!isset($unique_colors[$rgb])) {
            $unique_colors[$rgb] = $rgb;
        }
    }
}

sort($unique_colors);

echo '<div class="container">';
foreach($unique_colors as $index => $color) {
  echo '<div class="box" style="background-color:#' . $color . '"></div>';
}
echo'</div>';

// The function takes in an image resource (the result from one
// of the GD imagecreate... functions) as well as a width and
// height for the size of colour palette you wish to create.
// This defaults to a 3x3, 9 block palette.
function build_palette($img_resource, $palette_w = 3, $palette_h = 3) {
    $width = imagesx($img_resource);
    $height = imagesy($img_resource);

    // Calculate the width and height of each palette block
    // based upon the size of the input image and the number
    // of blocks.
    $block_w = round($width / $palette_w);
    $block_h = round($height / $palette_h);

    for($y = 0; $y < $palette_h; $y++) {
        for($x = 0; $x < $palette_w; $x++) {
            // Calculate where to take an image sample from the soruce image.
            $block_start_x = ($x * $block_w);
            $block_start_y = ($y * $block_h);

            // Create a blank 1x1 image into which we will copy
            // the image sample.
            $block = imagecreatetruecolor(1, 1); 

            imagecopyresampled($block, $img_resource, 0, 0, $block_start_x, $block_start_y, 1, 1, $block_w, $block_h);

            // Convert the block to a palette image of just one colour.
            imagetruecolortopalette($block, true, 1);


            // Find the RGB value of the block's colour and save it
            // to an array.
            $colour_index = imagecolorat($block, 0, 0);
            $rgb = imagecolorsforindex($block, $colour_index);

            $colour_array[$x][$y]['r'] = $rgb['red'];
            $colour_array[$x][$y]['g'] = $rgb['green'];
            $colour_array[$x][$y]['b'] = $rgb['blue'];

            imagedestroy($block);
        }
    }

    imagedestroy($img_resource);
    return $colour_array;
}