Grayscale

<?php

$r = json_decode($_POST['r'], TRUE);
$g = json_decode($_POST['g'], TRUE);
$b = json_decode($_POST['b'], TRUE);
$width = $_POST['width'];
$height = $_POST['height'];

$abu = [[]];

for ($y=0; $y < $height; $y++) {
    for ($x=0; $x < $width; $x++) {
        $abu[$x][$y] = round(($r[$x][$y] + $g[$x][$y] + $b[$x][$y]) / (float) 3);
    }
}

$img = imagecreatetruecolor($width, $height);
for ($y=0; $y < $height; $y++) {
    for ($x=0; $x < $width; $x++) {
        imagesetpixel($img, $x, $y, imagecolorallocate($img, $abu[$x][$y], $abu[$x][$y], $abu[$x][$y]));
    }
}

ob_start();
    imagejpeg($img);
    $contents = ob_get_contents();
ob_end_clean();

$base64 = "data:image/jpeg;base64," . base64_encode($contents);

$result = [
    'r' => $abu,
    'g' => $abu,
    'b' => $abu,
    'width' => $width,
    'height' => $height,
    'base64' => $base64
];
echo json_encode($result);