Headers, by definition, are sent before the page itself. As soon as the script produces any non-header output (including blank lines or spaces) any and all headers lined up ("Content-Type: text/html", for example) are immediately sent out first, followed by the non-header stuff (such as "<head><title>...").
If you then try and send another header, you get just such an error as you show.
What it's saying is that when PHP was processing line 9 of php_lib.php, it was told to output something. It fired off all the headers it had lined up and then outputted whatever it was. Later, on line 20 of LoginValidation.php, you tried to send another header. Too late: the headers have already been sent and we're on to the rest of the page now.
