I think there are a number of issues you should look at in your code.
Firstly, there is no point in checking for "defined('MOODLE_INTERNAL') || die();" immediately after including the main 'config.php' file (the MOODLE_INTERNAL check is for files that do not directly include config.php themselves, to avoid library files being accessed directly and causing unwanted side effects).
Next, if you want to do a simple query like you have done, use $DB->get_records('course'); instead of constructing an SQL query (and if you really need to construct an SQL query, make sure you write "{course}" not "mdl_course", or your code will break on any site that uses a non-standard prefix, including PHPUnit / Behat automated testing sites).
I'm not clear why you are calling $DB->get_records_sql($sql) and then ignore the result of that and immediately call $DB->get_recordset($sql). The first should work (although it would be better to use $DB->get_records(), as I mentioned above), but the second won't work, as it would expect the form: $DB->get_recordset('course', []); (you could use $DB->get_recordset_sql($sql) if you _really_ wanted to, but it's not the best approach).
Finally, you loop through $rs, returning each entry in the results as a variable called $record, before completely ignoring $record and trying to access a brand-new variable called $row (that you've not mentioned until then). $record will also be an object, so $record->id would be the correct way to access the id.
Please make sure you have turned
Debugging on to developer level and that will mean you get helpful error messages to show your mistakes.
