Thanks to Chris Suter for contributing the following tip:
I always knew this was in there somewhere, but usually my compiler tells me when I'm an idiot, so I've never needed it. Latex, however, does not.
:setlocal spell spelllang=en_us
turns on spell-check and highlighting for incorrectly spelled words.
]s - next misspelled word
[s - prev "
zg - add misspelled word to system dictionary
zG - add misspelled word to buffer-local set of "good" words
zw - add word to 'bad' list
zW - add word to buffer-local 'bad' list
z= - view a numbered list of suggested spellings from which to choose for auto-replacement
side note: Maybe there's a word list for php fn's, since it seems to assume that when you've mistyped something, you obviously meant 0, "zero", false and NULL simultaneously and chooses arbitrarily between them. This might prevent some headaches/suicides.