You can use addslashes( ) to make any input safe for comparisons/queries etc., such as:
$comment = addslashes( $_POST['comment'] );
...and then using stripslashes( ) when outputting the content later on.
A thing to note, when typing in (this) text box, line breaks are the invisible newline character, \n, so you'd need to replace them with <br/>:
$subject = str_replace( "\n", "<br/>", $subject );