如何避免带有大量换行符的文本？

I'm creating a forum for learning purposes and I have found a problem. Users can post a message (max 255 characters) but they also can send 255 characters with new lines so my forum appearance looks ugly, example:

How can I solve this? Any solution?

You can replace all line breaks with php: $newdata = str_replace(" " ," ", $data); $data is the string that users input.