I was placed on a PIP twice. Both times, I received no prior warning--official or otherwise--that my performance was poor whatsoever. HR simply showed up one day to a recurring 1:1 with my manager to inform me about the PIP. There are no official documented parameters or guidlines in place regarding PIPs or employee termination. The first PIP I was able to work off of successfully. The second PIP, it was very clear my manager simply wanted me gone and HR couldn't have cared less. Half of the PIP goals weren't even measurable, and half of the grading criteria wasn't shared with me until half-way through. My manager chose to change the requirements of the PIP three weeks in, something HR had no problem with, despite my reaching out to them for help multiple times. The new goals were again things I had never received prior warning that I was under-performing in, and I was given one week to improve these new goals. At the end of the PIP, I had objectively 100% met a third of the PIP requirements and had shown improvement in every other listed goal, except for one. This single goal was apparently weighed more than all others, something I was not told until the last week of the PIP. From what HR told me, partial improvement should have resulted in the extension of the PIP at the very least. It did not and I was terminated the very next day. The company does not give notices, and I had three hours before I lost access to the network. Every HR professional outside of the company that I've talked to since has told me that the entire process was poorly handled and extremely unprofessional by most HR standards. Every time I went to HR for help or support, they were dismissive and defaulted to my manager every time. The feeling of fighting and doing everything I could for four weeks to keep my job and have not a single person acknowledge that effort and dismiss all that work is indescribable.
tl;dr HR is not there for you, they are there for management only. If your manager doesn't like you, personally or otherwise, you're basically screwed.