What surname can an illegitimate child use?

Now, it is possible for illegitimate children to use the surname of their father.
Thanks to a new law enacted in 2004, Republic Act No. 9255 entitled "An Act allowing illegitimate children to use the surname of their father, amending for the purpose Article 176 of Executive Order No. 209, otherwise known as the Family Code of the Philippines."
Presently worded, Article 176 of the Family Code states:
"Illegitimate children shall use the surname and shall be under the parental authority of their mother, and shall be entitled to support in conformity with this Code. However, illegitimate children may use the surname of their father if their filiation has been expressly reconized by the father through the record of birth appearing in the civil register, or when an admission in a public document or private written instrument is made by the father. Provided, the father has the right to institute an action before the regular courts to prove non-filiation during his lifetime. The legitime of each illegitimate child shall consist of one-half of the legitime of the legitimate child.
The law brings relief for so many illegitimate children, who were once limited by to bear only the family of their mother.
There are so many births by mothers who chose not to marry, for one reason or another.
Many times, the father did not really object to having his family name used even if he is not married to the mother. But the law mandated then that the illegitimate child use only the surname of the mother.
Worse, in the birth certificate of the illegitimate child, in the space reserved for "Name of father" what is placed is "Unknown" even if the father is known and recognizes his child.
Requiring the birth certificate to contain an "unknown" entry is slanderous, and blackens the reputation of the mother. It suggests that the mother was engaged in an orgy when the child was conceived.
So, today, upon the birth of the illegitimate child, if the father reconizes the child as his, the birth certifiate will no longer contain the word "unknown" in the space reserved for the father's name.
Moreover, even if the father recognizes his illegitimate child years after the child's birth, the father can execute an affidavit admitting that he is the father of the child, or in a private handwritten document.