If you are willing to pursue PhD course after you finish your legal studies then you need to qualify in post graduation degree or complete M.Phil in Law in similar stream with a minimum aggregate of 55% from recognised university & then you can apply for the admission process. In your case, you can apply for doing PhD after completing LLM from Madras University.
The admission process to PhD course will be based on an entrance exam. So, after the results are out, you can pursue PhD if you have cleared the exam with the required marks.
Hope this helps.