string comparison on numerical array

I use the explode() function to split an article into words.so each element in the array is a word.how can i then do a string comparsion on this array to match a keyword provided in a search. i.e $article[$i] = $keyword; so $keyword is provided and i need to get the value of $i. i'm doing this in order to display a snippet or snippets of text containing the keyword. thanx.