I have the following code:
$message = \"Good news! The item# $item_number on which you placed a bid of \\$ $bid_price is now available for purchase at your b
I know this is an old question but anyway it might help someone.
I tend to use PHP_EOL for this purposes (due to cross-platform compatibility).
echo "line 1".PHP_EOL."line 2".PHP_EOL;
If you're planning to show the result in a browser then you have to use ".
"
EDIT: since your exact question is about emails, things are a bit different. For pure text emails see Brendan Bullen's accepted answer. For HTML emails you simply use HTML formatting.