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;
?>