Execute some logic before and after quiz attempt - moodle

Question

I am using "\mod_quiz\event\attempt_started" Event observers but i am not getting any result, php error messages or logs.
I am creating own local plugin for leader board functionality but I am stuck in Event observers call.
In events.php file
$observers = array(
array('eventname' => '\mod_quiz\event\attempt_started',
'includefile'=> '/local/leaderboard/locallib.php',
'callback' => 'quiz_attempt_started',
'internal' => 1
),
array('eventname' => '\mod_quiz\event\attempt_submitted',
'includefile' => '/local/leaderboard/locallib.php',
'callback' => 'quiz_submitted_start'
),
);
In locallib.php
function quiz_attempt_started($event) {
file_put_contents('/local/leaderboard/log.txt', $event);
//Logic Here
}
function quiz_submitted_start($event) {
file_put_contents('/local/leaderboard/log.txt', $event);
// Logic Here
}
With debugging on this is the error that moodle shows.
PHP Notice: Exception encountered in event observer 'quiz_attempt_submitted': Coding error detected, it must be fixed by a programmer: PHP catchable fatal error
- line 20 of \local\leaderboard\lib.php: call to default_error_handler()
- line ? of unknownfile: call to quiz_attempt_submitted()
- line 155 of \lib\classes\event\manager.php: call to call_user_func()
- line 89 of \lib\classes\event\manager.php: call to -
core\event\manager::process_buffers()
- line 2489 of \lib\dml\moodle_database.php: call to core\event\manager::database_transaction_commited()
- line 92 of \lib\dml\moodle_transaction.php: call to moodle_database->commit_delegated_transaction()
- line 2221 of \mod\quiz\attemptlib.php: call to moodle_transaction->allow_commit()
- line 88 of \mod\quiz\processattempt.php: call to quiz_attempt->process_attempt()
in E:\Moodle_Boost\03_Execution\04_Tech\03_Source Code\lms\lib\weblib.php on line 3096
Here noting to get error or result. Am I forgetting or doing something wrong here?