There have been a couple of threads posting regarding this error. Your host probably provides both PHP4 and PHP5. On these particular environments, if you're running PHP4, the server will still report you're running PHP5. The easiest option to fix the issue is to switch to PHP5. You should ask your host about that if you don't know how.
Alternatively, you could try the nightly build which has another check for PHP5. That might work better.
Edit: sbillard was quicker.