If the main array does not need to be sorted in any particuler order, you could do the following:
$random_array = shuffle($main_array);
$hundred_random_words = array_slice($random_array, 0, 100);
shuffle() shuffels the elements in the main array. array_slice() returns the a part of the array.
Of course there are other ways to do this, and its a bit unefficient to shuffle the whole main array, and then pick 100 words from it, but still it should work.
