All I can really tel you is this is extremely complex stuff.
I assume you have Googled:
http://www.google.ca/search?hl=en&rlz=1 ... sing&meta=
Natural Language Processing are the keywords of interest here.
It's not easy and not something I know of existing libraries that can help with, unlike parsing programming languages like PHP, I think natural languages have a much more flexible grammar, virtually limitless, which makes processing them very difficult.
English, unlike PHP, evolved over time, whereas, PHP's grammar was planned or already understood from the get go.
I suppose one could compile a database of all the caveats, etc of English, but I dought that is realistic, so I believe the field of NLP is basically taking educated/calculated guesses, otherwise if this field was mastered Google would be answering your questions perfectly.
My suggestion, would be to consider using something less trivial, like maybe soundex or download an English thesurasus to perform lookups on similar words. You can usually exlcude any words less than 3 characters.
Split the sentance up into words, drop those less than 3 characters, iterate array and soundex or compare to some dictionary source until you find something interesting.
Anything much beyond this is very theoretical and long winded, not to mention difficult to comprehend.