There are plenty of small editors like Notepad++, vim, & SciTE - but they only work on one file at a time. In other words, they aren't aware of the whole scope of the codebase.
On the other hand, all the full IDE's I have seen which offer code-completion (over all files in a project) take a good amount of memory to run. Eclipse PDT, PHPStorm, etc..
I'm looking for a tiny IDE with nothing more than code completion and syntax coloring to move around some very limit dev systems running PHP. Is there anything like this?
Windows or Linux
To clarify, if the app can't do multi-file code completion (complete code in one file, based on classes in other files) then it doesn't fit my requirements.
Geany is my editor of choice. It's small, cross-platform, (GTK) and supports literally dozens of languages. (including PHP) It has syntax highlighting, code-completion and even custom "build" commands. (such as php -l for sanity checks)