Fixed it. My mistake not understanding how to use the db abstraction. The issue was that I was trying to execute all that sql in one arrayQuery() as follows:
$row = $db->arrayQuery("
SET @row =0;
SELECT rank FROM (
SELECT @row := @row +1 AS rank, node_id, SUM( rate ) AS total
FROM ezarticle_rating
GROUP BY node_id
ORDER BY total DESC
) AS totp
WHERE node_id = '".$args['key']."';
" );
Splitting the two separate SQL statements (the first initialising the user variable) fixed it, like so:
$db->Query("SET @row =0;");
$row = $db->arrayQuery("
SELECT rank FROM (
SELECT @row := @row +1 AS rank, node_id, SUM( rate ) AS total
FROM ezarticle_rating
GROUP BY node_id
ORDER BY total DESC
) AS totp
WHERE node_id = '".$args['key']."';
" );
