What worked for me to convert a JPG source file to a semi-transparent PNG.
<?php
$srcImagick = new Imagick($srcPath);
$pathTo50Perc = 'transparent1.png';
$srcImagick->setImageAlphaChannel(Imagick::ALPHACHANNEL_OPAQUE);
$srcImagick->evaluateImage(Imagick::EVALUATE_DIVIDE, 2, Imagick::CHANNEL_ALPHA);
$srcImagick->writeImage($pathTo50Perc);
?>