To work in France, you need to have a work permit. For a PhD, you can ask for a “scientific” one, under the reference : CESEDA L 313-8.
Asking for a french work permit :
– pour les demandes de titre de séjour (1st demand or renewal), make your apointment at least 2 months before the end of your permit validity.
You have to fill the following file (it’s in french) : Demande de titre de séjour
You can see documents required for the demand (official document in french, but we’ll work on an english version soon) : Pièces à joindre à la demande
You can make an appointment to the prefecture foreign bureau online (your file must be complete) : www.rdv.tarn.gouv.fr/booking/create/702