How to solve possible toilet phobia in 3.9 yr old?

[SleepyRoo]

DD is 4 in June, starting school in Sept and has never managed a wee or poo on either a potty or toilet. For the last 6 months she will, with coaxing, do a wee in our bidet, with the water running, but nowhere else. She will only poo in a pull up, which she is given at night. At nursery she just holds it all in. Wears pants all day long, very rarely an accident. I've tried every bribe going but no luck. She becomes upset if we try to persuade her to sit on the loo (with toddler seat/step) to the point where I think she might actually be phobic. She has no other developmental delay, is articulate and bright, if anxious. There is no physical problem here, just psychological. Any advice on how I can gently help her move this situation forward? I have hit a brick wall.