Replying to Francois:
Imagick::ALPHACHANNEL_REMOVE has been added in 3.2.0b2 (before the RC): http://pecl.php.net/package-info.php?package=imagick&version=3.2.0b2
The problem is with people who want to implement this, but are stuck with an Imagick version < 3.2.0b2. They can't use this constant. However, all is not lost. I've found a reference where someone got this to work using an integer: http://stackoverflow.com/q/28154179/1000608
The number he used is 11, which seems to suggest that the value of Imagick::ALPHACHANNEL_REMOVE is 11, and the function worked correctly in this usecase even before the constant was implemented. So if you're stuck with <3.2.0b2 this is the code you need:
This works at least as far back as version 3.1.0~rc1-1 (current version of the php5-imagick package in Debian 7).