I have installed ImageMagick using
brew install imagemagick
Class 'Imagick' not found in ...
Your ImageMagick installation is not enough. You also need the Imagick package (possibly called
php5-imagick or similar for brew).
Imagick doesn't do the work itself, it is a native PHP extension for creating and modifying images using the ImageMagick API.
Try to search for the exact name of the package via
brew search imagick.