In addition to what everyone has said, it is important to know that there are two types of strings in PHP. Strings made with double-quotes (") and strings made with single-quotes ('). When you write
print 'This is my string.\n';
print '<br />';
print "This is my string\n";
You'll notice you get two different looking strings. The HTML output formatting characters (such as \n and \t and many others) only work with double-quotes. However, if you're like me, and use single-quoted strings more often to alleviate much of the difficulty debugging and diagnosing problems with a program, like for instance having to escape so many friggin double-quotes when I write HTML embedded in PHP code
print "<font size=\"12\" color=\"#000000\">I said, \"Hey!\"<br />";
Seriously, that looks disgusting.
You can have the best of both worlds by doing concatenating the string like this:
print 'This is my string.'."\n";
This will join the two types of strings together and allow you to format your webpage AND your HTML code in the same line.
Also, when formatting an email to send using PHP, if you're not using HTML formatting for your email, the standard newline character for emails is: \r\n
Again, to be used with double-quotes, not single-quotes.