I think it depends on what discipline you're in, I know several people who went straight from undergrad to PhD in the Biosciences, but it's less common in the environmental sciences are there are so many applicants. But you have an interview so they want to see you which is good, make sure you know the project background for the interview, be as confident as you possibly can about your skills and be really enthusiastic about wanting to submit publications and go to conferences etc.
There was a thread just a few days ago which was similar where someone I think Elmo? said they had applied for something like 13 PhD's before getting one, so don't give up if this one doesn't go as you hope, it's a tough market but it's possible!
Good luck!