chown uid, gid, files
Changes the owner and group of a list of files. The first two arguments must be the numerical uid and gid, in that order. The function returns the number of files successfully changed.
On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption.