I think nedals has asked the key questions, but just to try to anticipate - if you want the block of text divided into a fixed number of characters per line, that is easy, just looop through taking x characters from the beginning of the field until you have no more. More likely you want to break between words in which case the logic is something like
(to break into lines no more than x chars long)
while text left
start at character x
work backwards until you get to the beginning orfind the first space
if you are back at the beginning just take the first x chars (you will have to break it as they have entered a long string of text that wont fit)
if you are not at the beginning take off the characters up to the space you found
add a newline (\n)
Like nedals said though, this may or may not correspond to where the lines were automatically wrapped by the textarea control, this is purely a user interface thing and the position of the breaks is not passed to the server when the form is submitted.
Incidentally, again as nedals said, what you are going to use the wrapped text for is critical, if you are going to put it in an email for example then the above method of chopping it into lines makes sense, but if you are going to show the message in a browser, you would probably be better letting the browser do the wrapping itself...