update: just found an easier way to create a transparent canvas 
so, you can pretty much ignore all these options listed below now
$im = new Imagick();
$im->newImage(100, 200, new ImagickPixel('transparent')); // use this predefined transparent color string
$im->setImageFormat('png32');
//$im->setImageFormat('gif');
//$im->setImageFormat('jpg'); // note that jpg does not support transparency, so you see a default black background
==== ignore the rest of the post ====
According to ImageMagick Examples canvas page, there are multiple ways to create a transparent canvas.
It is a pity that Imagick PHP library lacks enough documentation, so I had to try my luck with it. For example, I want to create a transparent background, then copy an image with transparency on top of it.
< ?php
try
{
$icon = new Imagick('play.png');
$im = new Imagick();
// put code here to create the transparent canvas, use options below
$im->compositeImage($icon, Imagick::COMPOSITE_COPY, 0, 0);
header(’Content-Type: image/png’);
echo $im;
}
catch (Exception $ex)
{
echo ‘Exception: ‘ . $ex->getMessage() . “\n”;
echo $ex->getTraceAsString();
}
?>
a) setImageOpacity
$im->newImage(200, 200, '#000000', 'png');
$im->setImageOpacity(0.0);
b) paintTransparentImage
$transparent = new ImagickPixel('#000000');
$im->newImage(200, 200, '#000000', 'png');
$im->paintTransparentImage($transparent, 0, 10);
c) matteFloodfillImage
$transparent = new ImagickPixel('#000000');
$im->newImage(200, 200, '#000000', 'png');
$im->setImageBorderColor($transparent);
$im->matteFloodfillImage(0.0, 0.0, $transparent, 200, 200);
d) thresholdImage
$im->newImage(200, 200, '#000000', 'png');
$im->thresholdImage(-1, Imagick::CHANNEL_ALPHA);
e) fxImage
$im->newImage(200, 200, '#000000', 'png');
$im = $im->fxImage('0', Imagick::CHANNEL_ALPHA);
For ImageMagick 6.2.9 12/17/07 Q16, Imagick 2.2.2RC1
a) does not work at all
e) gives the correct result, transparent background for play.png and transparent background for the whole image
others give transparent background for play.png, but black background for the whole image
For ImageMagick 6.4.9-6 2009-03-25 Q16, Imagick 2.2.2
a) b) gives transparent background for play.png and whole image
c) gives an exception
others give white background for play.png, black background for whole image
Images (note that you may want to save the images and open it in a viewer with non-white background to see the difference, especially the last two):
1. Desired ouptput: transparent play button background and transparent canvas:
2. White play button background and black canvas
3. Transparent play button background and black canvas
