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

    $draw->setStrokeColor($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();
}