Example

function getPixelRegionIterator($image_path)
{
    $imagick = new \Imagick(realpath($image_path));
    $imageIterator = $imagick->getPixelRegionIterator(100, 100, 200, 200);

    /** @noinspection PhpUnusedLocalVariableInspection */
    foreach ($imageIterator as $row => $pixels) { /* Loop through pixel rows */
        foreach ($pixels as $column => $pixel) { /* Loop through the pixels in the row (columns) */
            /** @var $pixel \ImagickPixel */
            if ($column % 2) {
                $pixel->setColor("rgba(0, 0, 0, 0)"); /* Paint every second pixel black*/
            }
        }
        $imageIterator->syncIterator(); /* Sync the iterator, this is important to do on each iteration */
    }

    header("Content-Type: image/jpeg");
    echo $imagick;
}