Is a child obligated to honor his foster parents? Does it matter if the child cannot locate his biological parents?
The commandment of kibbud av v'eim does not apply to adoptive parents (Sotah 49a - right before the Mishna). However, there is a moral obligation of hakarat hatov (gratitude) which requires that the child honor his/her adoptive parents.
(Source. The article states that "one must honor his/her adoptive parents as much as if they were the biological parents." However, the Talmud in Sotah says pretty clearly not that way.)