When I use the DOMDocument class in my PHP project, it runs as expected, but the IDE does not recognize the class, so I have no code assistance.
I cannot figure out why this is; do I have to include anything special?
What PhpStorm version do you use exactly (Help | About) ?
Please try "File | Invalidate Caches...".
If it will not help -- please provide some small but complete code example to reproduce (code to copy-paste .. and maybe screenshot of the error message).
Invalidate Caches did the tric: apparently the index was not up-to-date.
(I use 5.0.4, the most recent version).