I recently installed some PHP site on a server for a customer. A guy calls me up and tells me he can't seem to upload to the img directory. I went in and took a look at the owner of said directory.  The owner was set to a user named 518. I was still able to chown -R someuser img/ and fix the issue, but I am more concerned that the owner of the directory was set to 518. I have never seen the owner set as a number and I certainly did not set that owner myself. What does it mean when a user is set to 518? Did I get broken into? When I run cat /etc/passwd I do not see any user named 518 in there. I am running Ubuntu.
Owner of directory set to 518?
ubuntu;chown
When you see a numeric UID rather than a name it means simply that your host cannot resolve that UID. E.g. there is no entry in the nameservice. That is all. Quite where it came from is an entirely different mystery, and not easy to answer without more detail. I would start by checking your LDAP directory, or other name service to see if there is a UID 518 defined. A common way of having this sort of problem though, is unpacking a 'tar' archive, which retains UID/GID information from the source, by default. 