DE CASTRO vs DE CASTRO

The validity of a void marriage may be collaterally attacked.

In Niñal v. Bayadog, the court held:
However, other than for purposes of remarriage, no judicial action is necessary to declare a marriage an absolute nullity. For other purposes, such as but not limited to determination of heirship, legitimacy or illegitimacy of a child, settlement of estate, dissolution of property regime, or a criminal case for that matter, the court may pass upon the validity of marriage even in a suit not directly instituted to question the same so long as it is essential to the determination of the case. This is without prejudice to any issue that may arise in the case. When such need arises, a final judgment of declaration of nullity is necessary even if the purpose is other than to remarry. The clause "on the basis of a final judgment declaring such previous marriage void" in Article 40 of the Family Code connotes that such final judgment need not be obtained only for purpose of remarriage.

Illegitimate children may establish their illegitimate filiation in the same way and on the same evidence as legitimate children. Thus, one can prove illegitimate filiation through the record of birth appearing in the civil register or a final judgment, an admission of legitimate filiation in a public document or a private handwritten instrument and signed by the parent concerned, or the open and continuous possession of the status of a legitimate child, or any other means allowed by the Rules of Court and special laws.