You can study a Doctor of Human Resources, Management or Organisational Psychology.
Your PhD in this field can help you develop a thesis related to your chosen field of interest, whether it be employment laws, global human resource management, strategic human resource management or something else.
Most companies will not require that their human resources staff have a doctorate before they hire you, but there is no denying that your employment options will expand with a PhD under your belt. It shows true commitment to your career, and that you are an expert in the field of human resources.
If you study your doctorate full-time, you should be able to receive some funding. There are also many university sponsored human resources scholarships which you may be eligible for. You can review them all here.
Generally, people will require a bachelor's or master's degree in human resources, or a related field, before they can commence studying their doctorate. Many educational institutions offering a PhD in human resources would also prefer you to have some years of professional experience, and a basic outline about what you want to write your thesis on.