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Unfortunately, unless you can allege that the basis for this treatment of you is your race, religion, gender, age, disability or FMLA use (medical leave), the harassment itself is not illegal. The Supreme Court made it clear when defining hostile work environment that our employment laws are not meant to be a civility code and without the ingredient of discrimination based on those factors I mentioned, there is no claim of hostile work environment.
Specifically to the PIP, unless you have an employment contract that expressly states that you can only be terminated for cause, your employment is "at will." That means that it can legally be terminated at any time, with or without cause. So, placing you on a PIP for a weak or really unreasonable reason isn't illegal either. Being belittled in front of others, while rude and unprofessional, isn't illegal.
Without some additional factor to make these facts illegal, unfortunately all I can tell you here is that the employer has not broken the law.