I would highly recommend a VPS. While a high-end shared account might suit your needs at this time, I think that given your site's growth rate, your site will soon outgrow it.

Depending on how heavy the PHP software is, I would recommend a VPS with 128-256 of dedicated RAM. Everything else is pretty basic - you'd need enough space and bandwidth to allow for growth for the next 6 months or so. I'd also suggest not to lock yourself on an any annual terms. But you can get extra savings if you go with quarterly payments.

You can always find a lot of great deals/discounts in this forum:
http://www.hostingdiscussion.com/vps...pecial-offers/