I am confused why the following code returns two Xs between each paragraph. I was expecting only one.

$paragraphs = str_replace("\n", X", $paragraphs);

So then I figured there must be two line breaks in the text coming from the database. But when I tried the following code, there was no X at all between each paragraph.

$paragraphs = str_replace(\n\n", X", $paragraphs);

Im clearly not understanding about something regarding line breaks. If it makes any difference, Im working on a Mac, and I think they handle line breaks differently than a PC.