word_wrap() bug???

[eluser]Cristian Gilè[/eluser]
I'm trying to use word_wrap() function but the text is not wrapped.
I've used the example in the CI guide:

Code:

$string = "Here is a simple string of text that will help us demonstrate this function.";

echo word_wrap($string, 25);

// Would produce:

Here is a simple string
of text that will help
us demonstrate this
function

Can you explain that ??
Is it a potential candidate bug ??