if you put your strings in double quotes you can use \n for a new line. this however does not produce a <br /> as this is html and nothing to do with php, but there is a function [man]nl2br[/man] that will then convert all your \n's into <br /> tags.
example;
<?php
$mystring = "this is a line\nand this is a new one\n";
echo $mystring;
echo nl2br($mystring)
?>
would produce;
this is a line
and this is a new one
this is a line<br />and this is a new one<br />