I believe 1&1 still uses PHP4 on their plans by default. There is documentation on their site about how to switch to PHP5. Be careful with them, they limit databases to a maximum of 100 megabytes.
I've seen a lot of people bit by bargain-basement shared hosting, including HostGator and 1&1. Always make sure to read the TOS for a hosting company to know where those limits on "unlimited" are. If you plan to get a lot of traffic, also be careful because SilverStripe can be heavy at times, and many shared hosts automatically suspend sites with heavy CPU usage.
Furthermore, avoid any companies owned by Endurance International Group (BizLand, FatCow, PowWeb, and iPowerWeb among 20+ brands they market under). I've personally experienced the disservice from one of their brands, and have heard horror stories from those who have used their other brands.
I've heard very good things about [url=http://www.montecarlohosting.net/]MonteCarloHosting[/url], and their resource allocations are quite reasonable for the price.