Ask HN: matrix multiplication within PHP?

I am implementing a PHP-based web site that includes a simple recommender system. The key numerical calculation is a matrix multiplication. I am worried that doing this in PHP might be painfully slow. Does anyone else have experience doing numerical computations in PHP? I looked at writing my own PHP extension ( http://www.tuxradar.com/practicalphp/20/0/0 ) to implement matrix computations in C, ... but then it struck me someone else must have already encountered and solved this problem.

Any recommendations?