Here is line 566 of the moodlelib.php
function optional_param($parname, $default, $type) {
if (func_num_args() != 3 or empty($parname) or empty($type)) {
throw new coding_exception('optional_param() requires $parname, $default and $type to be specified (parameter: '.$parname.')');
Here is line 12 of the backup.php
$id = optional_param( 'id' ); // course id