Beware that fxImage return value's type is a new object(Imagick), opposing the current documentation (boolean). Therefore it does not affect the caller object. Example:
<?php
            $canvas = new Imagick();
            $canvas->newImage(256, 256, "white", "jpg");
            $gradient = new Imagick();
            $gradient->newPseudoImage(256, 256, "gradient:white-black");
            $canvas->compositeImage( $gradient, imagick::COMPOSITE_OVER, 0, 0 );
            $canvas->rotateImage(new ImagickPixel(), 90);
            $canvas= $canvas->fxImage("floor(s*10+0.5)/10");
            header( "Content-Type: image/jpg" );
            echo $canvas; 
?>