Should I include proof of my extenuating circumstances when applying for a PhD? My performance in my bachelor's degree was adversely affected by some medical extenuating circumstances (related to physical injuries). I am currently in a master's program and my performance is much better. Should I mention these extenuating circumstances in my PhD applications and should I include the relevant documents (doctors' statement, diagnostic report, etc)?
You might mention extenuating circumstances if:

They significantly decreased your past performance.
You explain why your performance will not be reduced in the future for the same reason.

Do not include any evidence unless it is requested, or perhaps if the circumstances were something very unlikely.  Having been imprisoned for a crime and later exonerated might be an example where evidence would be a good idea.