English certifications: which ones are recognized overseas?
From what I've heard, the TOEFL is well recognised. 

That being said, once you get your PhD, I don't think people ask for some English certifications (at least, I've never been asked to, and I'm not a native speaker). I guess your publications and the interview in English should be enough to see if you're able to communicate in English. 

EDIT List of who accepts the TOEFL.