I have an invisible but significant disability and I have been considering my interview options quite a bit lately. As I see it there are two courses of action.
First option is that you don't say anything and just get on with answering the questions. Direct questions about your disability are likely to be few, because of sensitivities about their legal position, and how much your disability might seem to influence their decision - quite clearly it shouldn't influence. The advantage is that is might appear that your disability is not relevant or significant. The disadvantage is that your interviewers may not think like this and you may not know this until too late.
The second option is to be entirely up front and get the possible issues out of the way as soon as possible. A bold, 'By the way, please ignore my squint, I know it appears as if I'm not making eye contact, but I can assure you I'm going to be listening closely to you!' as you settle intothe start of the interview shows that you are coping and also aware of the impact of your disability. I'm aware of the 'ism' and 'ists' of disability politics that may say that you shouldn't have to take option two and should be able to just get on with life, however, back in the real world.....
Also, don't forget that a PhD interview is much more focussed on your intellectual ability than a job interview which sweats more about the office environment, customer interface, overseas travel etc. So long as your data doesn't require you to climb mountains etc then you should be able to rely on your intellect to be the focus of everyone's attention.
My circumstances are slightly different, but I'm basically going to take option two, explain the problem and my coping mechanism up front and then move on.