Image ratio?

If I had an images height and width as $height and $width, how could I check to see if an images ratio is bigger than 1 : 10 using the height and width?

I want to do this so someone can’t upload an avatar which could have dimensions of 10x1000 etc.

I think I worked it out…

$ratio = ($width * 10) / $height;

if ($ratio < 1)

Need to work on my maths XD.