Is seems that at least for SQL deletes, you should pass a MySQL link resource (from a mysql_connect call), not a MySQL result (from a mysql_query) otherwise you will get an error.
(PHP 4, PHP 5, PHP 7)
msql_affected_rows — Returns number of affected rows
$result
) : int
Returns number of affected rows by the last SELECT, UPDATE or DELETE
query associated with result.
Returns the number of affected rows on success, or FALSE on error.
Is seems that at least for SQL deletes, you should pass a MySQL link resource (from a mysql_connect call), not a MySQL result (from a mysql_query) otherwise you will get an error.