In Imagemagick, there is no such command as -backgroundcolor. And Imagick has no command setBackgroundColor. It is setImageBackgroundColor.
I think the latter is equivalent to the command line -background somecolor?
-background is a setting, not an operation. It needs to be used by another option such as -extent.
You cannot change the background color of an existing image by just using -background color.
Lets say you have an image with a large canvas of red and in the middle you have a small square of blue. Using -background color to change red to green with do nothing. You would need to use the equivalent of -fuzz XX% -fill green -opaque red. See http://www.imagemagick.org/Usage/color_basics/#replace
If all you are trying to do is create an image of some flat color, then you probably need to do that with the creation of the image object. See http://www.php.net/manual/en/imagick.newimage.php
Perhaps I misunderstand what you are trying to do. Can you be a bit more clear about what you are trying to do. Also what version of IM, what platform, what version of Imagick?