When applying for PhD in US or EU, do universities give any importance to the summer schools (7 days/15 days) candidate has attended? I mean, are the summer schools a worthy addition to the CV?
If you did something relevant that relates to the PhD position you are applying to, then it should be fine to include it in the CV.
My contacts in the chemistry department at my institution have told me that "summer schools" don't really count that much, and that your research experience/publication record, letters of recommendation, and coursework rigor/performance are worth much much more.
Let me tell a story. A certain institution has been putting on a "summer school" for graduate students in a certain branch of mathematics for a few years now. One of the graduate students from my institution went one year. He told his advisor that quite a few students attending treated it like a paid-for vacation (indeed, I thought the attendees were paid an obscene amount to such sit in on a few lectures upon finding out how much they were paid). Those people simply didn't show up to the lectures or, if they attended, were just doing their own thing (reading something else, on their laptops, sleeping, etc.) during the lectures.
So it would make sense, at least to me, there would (and should) be little to no weight put on a summer school that consisted of not requiring any 1-on-1 interaction with faculty nor any sort of submitted written work on the part of the students. Or not requiring doing anything in a lab, in this case I suppose.
Certainly it doesn't hurt to include a summer school on your CV though, I just can't see it helping all that much either.