PIP does not count as taxable income, so it has no effect on the amount of tax you pay. In this regard, PIP is the same as DLA, which is also tax-free income.
Also, PIP, like DLA, is payable whether you are in or out of work and regardless of how many hours you work or how much you earn. However, any physical or mental activities involved in your employment can be taken into account when deciding how many points you score under the PIP assessment.