In the Philippines, the surname of a child is governed by the Family Code of the Philippines and Republic Act No. 9255. The requirements and procedures for changing a child's surname to that of the father depend entirely on the child's legitimacy status at the time of birth and whether the father has formally recognized the child.

1. Legitimate and Legitimated Children

Under Article 174 of the Family Code, legitimate children—those born during a valid marriage—are required by law to principally use the surname of the father.

Legitimation: If a child was born out of wedlock to parents who were free to marry at the time of conception, and the parents subsequently marry, the child is "legitimated."
Process: Upon the marriage of the parents, a Registration of Affidavit of Legitimation is filed with the Local Civil Registrar (LCR). The original birth certificate is not destroyed, but an annotation is made, and a new birth certificate is issued reflecting the father's surname.

2. Illegitimate Children (R.A. 9255)

Historically, illegitimate children were required to use the mother’s surname. However, Republic Act No. 9255 (amending Article 176 of the Family Code) allows illegitimate children to use the father's surname if the father has expressly recognized the child.

Recognition Requirements

To change the surname to the father's, one of the following must be presented:

Affidavit of Admission of Paternity (AAP): A public document executed by the father admitting paternity.
Private Handwritten Instrument (PHI): A document written and signed by the father in his own handwriting, clearly admitting he is the father.
Affidavit of Acknowledgement: Usually found on the back of the Birth Certificate (Certificate of Live Birth).

The Affidavit of Use of the Surname of the Father (AUSF)

Even if paternity is admitted, the surname does not change automatically. Under Revised IRR of R.A. 9255, the AUSF must be executed:

If the child is 0–6 years old: The mother or guardian executes the AUSF.
If the child is 7–17 years old: The child executes the AUSF with the mother’s attestation.
If the child is of age (18+): The child executes the AUSF personally, without need for the mother’s consent.

3. Procedure for Filing

The application is filed at the Local Civil Registry Office (LCRO) where the birth was recorded. If the birth occurred abroad, it is filed with the Philippine Foreign Service Post.

Standard Document Checklist:

Certified True Copy of the Birth Certificate (PSA copy).
Affidavit of Admission of Paternity (if not already signed on the birth certificate).
Affidavit of Use of the Surname of the Father (AUSF).
Valid IDs of the signatories.
Certificate of Registration (issued by the LCR after the petition is processed).

4. Judicial Petition for Change of Name (Rule 103)

In cases where R.A. 9255 does not apply—such as when the father refuses to sign an admission of paternity or there is a dispute regarding the child's identity—the parties must file a Petition for Change of Name under Rule 103 of the Rules of Court.

Venue: The Regional Trial Court (RTC) of the province where the person resides.
Grounds: The petitioner must prove that the change is necessary to avoid confusion, or that the child has been known by the father's surname since infancy.
Nature: This is a summary judicial proceeding that requires publication in a newspaper of general circulation for three consecutive weeks.

5. Key Limitations and Jurisprudence

Consent: If a child is of age, the mother cannot force the use of the father's surname; the choice lies with the child.
Illegitimacy Status: Using the father's surname does not change the child's status from "illegitimate" to "legitimate." Only a valid marriage between the parents (legitimation) or a court decree of adoption can change a child's legal status.
The "Shall" vs. "May" Rule: The Supreme Court (e.g., in Grande v. Antonio) has clarified that while R.A. 9255 gives the child the right to use the father's surname, it is not mandatory. The mother or the child (if of age) maintains the discretion to keep the mother’s surname if they so choose.