(PECL imagick 2 >= 2.1.0, PECL imagick 3)
Imagick::setImageAlphaChannel — Sets image alpha channel
$mode): bool
   Activate or deactivate image alpha channel. The mode 
   is one of the Imagick::ALPHACHANNEL_* constants.
   This method is available if Imagick has been compiled against ImageMagick version 6.3.8 or newer.
  
mode
       One of the Imagick::ALPHACHANNEL_* constants
      
   Returns true on success.
  
Throws ImagickException on error.