Sets the opacity to use when drawing using the fill color or fill texture. Fully opaque is 1.0.
function setFillAlpha($strokeColor, $fillColor, $backgroundColor)
{
    $draw = new \ImagickDraw();

    $draw->setFont(__DIR__ . "/../../../fonts/Arial.ttf");
    $draw->setStrokeColor($strokeColor);
    $draw->setFillColor($fillColor);
    $draw->setStrokeOpacity(1);
    $draw->setStrokeWidth(2);
    $draw->rectangle(100, 200, 200, 300);
    @$draw->setFillAlpha(0.4);
    $draw->rectangle(300, 200, 400, 300);

    $imagick = new \Imagick();
    $imagick->newImage(500, 500, $backgroundColor);
    $imagick->setImageFormat("png");
    $imagick->drawImage($draw);

    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}