The different image types that can be specified are (from the Imagick::getImageType doc page):
Imagick::IMGTYPE_UNDEFINED
Imagick::IMGTYPE_BILEVEL
Imagick::IMGTYPE_GRAYSCALE
Imagick::IMGTYPE_GRAYSCALEMATTE
Imagick::IMGTYPE_PALETTE
Imagick::IMGTYPE_PALETTEMATTE
Imagick::IMGTYPE_TRUECOLOR
Imagick::IMGTYPE_TRUECOLORMATTE
Imagick::IMGTYPE_COLORSEPARATION
Imagick::IMGTYPE_COLORSEPARATIONMATTE
Imagick::IMGTYPE_OPTIMIZE
The MATTE variants are equivalent to GrayscaleAlpha, etc. in the ImageMagick command line. If you're making something grayscale, I would recommend using IMGTYPE_GRAYSCALEMATTE.
<?php
$im->setImageType(Imagick::IMGTYPE_GRAYSCALEMATTE);
?>