Why not just use built in php functions like: substr and explode?
for example, if you want to separate the words in the string into an array do:
$yourarray = explode(" ", $mainstring); // This will make an array of the words because it's splits the string wherever there's a space (" ").
if you want to take a chunk out of the string do:
$chunk = substr($mainstring, a, b);
//a would be a number for which letter you want to start at.
//b would the number of characters until the chunk ends.
eg. $chunk = substr($mainstring, 2, 6);
this (using your string above) gives "is is "\
hope this helps, if not, check out the manual for both functions here:
http://us3.php.net/manual/en/function.explode.php
and
http://us3.php.net/manual/en/function.substr.php