To register for PhilHealth membership, senior citizens typically need to provide a valid ID, such as a passport or government-issued identification, to verify their age. Additionally, they may need to submit a completed PhilHealth Member Registration Form, which can be obtained from the PhilHealth website or local offices. It's advisable to also have a copy of their birth certificate or any document showing their date of birth. For the most accurate and updated information, check with PhilHealth or visit their official website.