The Explode function will allow you to split a string up by the string that you declare. So you could split a paragraph up into sentences by splitting it by a full stop or even split a sentence up into words by splitting it by a space. This video will show you how to explode a string and then access each string stored inside arrays.
For more information, check out the website:
http://howtostartprogramming.com/PHP/php-tutorial-20-explode/
