PHP fwrite new line

Use PHP_EOL which produces \r\n or \n

$data = 'my data' . PHP_EOL . 'my data';
$fp = fopen('my_file', 'a');
fwrite($fp, $data);
fclose($fp);

// File output

my data
my data

You append a newline to both the username and the password, i.e. the output would be something like

Sebastian
password
John
hfsjaijn

use fwrite($fh,$user." ".$password."\n"); instead to have them both on one line.
Or use fputcsv() to write the data and fgetcsv() to fetch it. This way you would at least avoid encoding problems like e.g. with $username='Charles, III';

...i.e. setting aside all the things that are wrong about storing plain passwords in plain files and using _GET for this type of operation (use _POST instead) ;-)