PHP MySQL Transaction wrong example

<?php
    $DB->query("SET AUTOCOMMIT=0");
    $DB->query("START TRANSACTION");
    // Check of exists
    $q = $DB->query("SELECT * FROM `test_overload` WHERE `code`='{$code}'");
    if( $q->num_rows == 0 )
    {
        $DB->query($DB->insert_string('test_overload', $data));
        $count_insert++;
    }
    else
    {
        $DB->query($DB->update_string('test_overload', $data, array('id'=>$q->row['id'])));
        $count_update++;
    }
    $DB->query("COMMIT");
?>