when i try
echo "anything \n whatever";
<h1>Authentication v 0.04</h1>
<form action="" method="GET">
<input type="text" name="username" /><br/><br/>
<input type="submit" value="connect" /><br/><br/>
$_GET['username'] = "anything \n whatever"
"anything \n whatever"
\n is an escape sequence which represents an end of line . An escape sequence only makes sense on PHP interpolation (i.e. double quotes).
\n has no special meaning so it is printed as a string. 
If there is an actual new line, you may want to use the
nl2br function (http://php.net/manual/en/function.nl2br.php) to automatically convert any
\n to a
If you want to convert any literal
<br>, you may want to use a
str_replace function (http://php.net/manual/en/function.str-replace.php) for example like this:
echo str_replace ('\n', '<br>', $_GET['username']);
Notice the single quotes to avoid further interpolation.
 In fact, it is a new line only on *nix systems, while it's different on Microsoft and old Mac OSx. You may want to take a look at
 In fact, it is printed as a new line in the HTML source, which is a text file.