Family Code, Art. 172

The filiation of legitimate children is established by any of the following: (1) The record of birth appearing in the civil register or a final judgment; or (2) An admission of legitimate filiation in a public document or a private handwritten instrument and signed by the parent concerned. In the absence of the foregoing evidence, the legitimate filiation shall be proved by: (1) The open and continuous possession of the status of a legitimate child; or (2) Any other means allowed by the Rules of Court and special laws. (265a, 266a, 267a)