The entrance exam for a PhD varies depending on the country, institution, and field of study. Common exams include the GRE (Graduate Record Examination) in the United States, GATE (Graduate Aptitude Test in Engineering) in India for engineering disciplines, and CSIR-UGC NET (Council of Scientific and Industrial Research - University Grants Commission National Eligibility Test) for sciences in India. Other countries may have their own national exams or require specific subject tests. Some universities also conduct their own entrance exams to assess candidates' research aptitude and subject knowledge. It's essential for prospective PhD candidates to research and prepare for the specific exams required by the institutions and Phd programs they are interested in.