Removed in ImageMagick 7. Imagick::RotationalBlurImage gives a very similar result.
function radialBlurImage($image_path)
{
    $imagick = new \Imagick(realpath($image_path));
    $imagick->radialBlurImage(3);
    $imagick->radialBlurImage(5);
    $imagick->radialBlurImage(7);
    header("Content-Type: image/jpeg");
    echo $imagick->getImageBlob();
}