Thanks for the help! I had downloaded Autocomplete, but that wasn’t really what I was looking for, and I looked for wordcomplete to download, but I couldn’t. So I did a little digging and found that it’s a part of the default installation, but my install didn’t include them for some reason. So I uninstalled, and redownloaded the installer and now a whole list of plugins show up, including wordcomplete.
Though a perfect wordcomplete would only show suboptions (i.e. HOST_NAME, etc for $_SERVER), I can live with this. ;]