From Good-Tutorials: PHP –Limiting text by a number of characters, cutting off at the end of a word. (strrpos).:
If you need to display source text, but you only want to show an excerpt of it and you will probably use the function strrpos to cut your text by a number of characters. This can look ugly as it cuts the text mid-string. Beneath is a tutorial showing you how to cut your text by a certain amount of characters and at the end of the word.