Given a string, will extract keywords from it. Skipping SGML tags and stemming words (e.g., "running" becomes "run", but "bring" is not changed), the unique list of keywords are placed in the keywords as an array. Also, some common english words (the articles, "for" "are", etc.) are removed. Note that there are some words may stem unexpectedly (e.g., "has" transforms to "ha").
keyword_extract( string, keywords var )
Populates the keywords array and returns number of elements.