Actually that is not functionality that parse_str provides. Parse string will parse a query string type string into variables, ie:
$strNames = "first=fred&last=flintstone";
parse_str($strNames);
You would now have to variables in the current scope called $first and $last with their appropriate values. With an array as the second parameter, it will just put those values into an array instead of variables.
The function you are looking for is split().
$output = "me and you go flying";
$values = split(" ",$output);
$values[2] now equals "you".
G'luck,
Jack