Try this instead.
<?php
echo 'Line 1<br />';
echo 'Line 2<br />';
?>
the '\' is a special character, the escape character, when used in a double quoted string or heredoc's.
html for a line break is <br> or <br />
If you dont expect to use variables or special characters in a string use the single quote. It stops mistakes and is self documenting.
Have a quick read on this page:
http://php.net/manual/en/language.types.string.php