Not sure if this is an issue with Atom or Emmet.
After installing Emmet via apm, firing up atom and then with the default untitled file open, I typed in html:5 to test it out. Unfortunately, that abbreviation ( or any abbreviation ) that Emmet supports is not being expanded. You have to save the file first to .html before Emmet works.
If I save the file with a .php extension, expand abbreviation also doesn’t work. With the example above, it outputs to