To enrol in a PhD programme you usually need either very good English or very good German skills.
PhD programmes in Germany have no standard language requirements. University institutes have their own regulations (called 'Promotionsordnung') and they vary from university to university. Most universities will accept a IELTS or a TOEFL certificate. Some will not require any certificate at all, if the applicants are either English native speakers or if they acquired a degree in a programme that was taught in English.
If you have a particular university in mind, we suggest you contact the administration team directly and ask them what language requirements they have. If you are still deciding on a PhD programme then you can search through international degree programmes in the DAAD online database.
This database allows you to search for your research field of interest. You will also see if you need a particular certificate. Most programmes are taught completely in English and each programme has a coordinator who you can contact for more detailed information.