How about:
$search_string = '"the color purple" movie soundtrack';
$search_terms = array();
$search_terms = preg_match_all ('/"[^"]+"|\w+/', $search_string, $matches);
print_r ($matches);
Array
(
[0] => Array
(
[0] => "the color purple"
[1] => movie
[2] => soundtrack
)
)