I found Imagick::clipPathImage and Imagic::clipImage did not work as I had expected. I thought they would just clip the path and throw away the extra data and you are done. Not the case.
Here is how I was able to use a clipping path:
$img = new Imagick("/Path/To/Test/Image.psd");
$geometry = $img->getImageGeometry();
// Uses the first path as the clipping path
// Fill the clipped part with a color
$draw = new ImagickDraw();
// Composite the clipped image with the old image. Set the color of the composite to any color you want to be the outside part.
$composite = new Imagick($path);
$composite->newImage( $geometry['width'], $geometry['height'], new ImagickPixel("white"), 'png');
$composite->compositeImage($img, imagick::COMPOSITE_COPY, 0, 0);
Then doing any resizing or creating thumbnails from the resulting image disregarded all the previous commands so I "saved" it and started with a new Imagick object
// Copy the image so clip is "saved"
$clipped = new Imagick();
I'm sure there is a simpler way, but this took me awhile to get right and there were some hurdles to cross so I hope it is able to help someone on the way.
This is all the convert equivalent of:
$ convert Test.psd -fill white -colorspace rgb -draw "color 0 0 reset" -clip -colorspace rgb -draw "Image Copy 0,0 0,0 'Test.psd'" OutputFile.png
(PECL imagick 2.0.0)
Imagick::clipPathImage — Clips along the named paths from the 8BIM profile
Clips along the named paths from the 8BIM profile, if present. Later operations take effect inside the path. It may be a number if preceded with #, to work on a numbered path, e.g., "#1" to use the first path.
The name of the path
TRUElater operations take effect inside clipping path. Otherwise later operations take effect outside clipping path.
TRUE on success.
Throws ImagickException on error.