Replace all pixels of one color with pixels of another color

pixel_replace.php

<?php
$img = \imagecreatefromjpeg('someimage.jpg');
for ($y = 0; $y < imagesy($img); $y++) {
    for ($x = 0; $x < imagesx($img); $x++) {
        // find color index at x,y
        $pixelColorIndex = imagecolorat($img, $x, $y);
        $pixelColorRGB = imagecolorsforindex($img, $pixelColorIndex);

        // set new color if test color matches
        if ($pixelColorRGB['red'] == 255 && $pixelColorRGB['green'] == 255 && $pixelColorRGB['blue'] == 255) {
            $newPixelColorIndex = imagecolorallocate($img, 231, 230, 230);
            imagesetpixel($img, $x, $y, $newPixelColorIndex);
        }
    }
}
imagejpeg($img, 'somenewimage.jpg', 100);