Mysqli

$filepath = $this->storage_path . 'marketing_opt_out.csv';

$lookupSql = 'SELECT id FROM orderwise_subscribers WHERE orderwise_id = ?';
$insertSql = 'INSERT INTO orderwise_subscribers (orderwise_id, subscribed, email, status, import_into) VALUES (?, ?, ?, ?, ?)';

$status = Subscribers::STATUS_READY_TO_IMPORT;
$importInto = Subscribers::IMPORT_INTO_DM;

if($fp = fopen($filepath, "r")) {
    $y = 1000;
    while ($csvLine = fgetcsv($fp)) {
        $handle = $mysqli->query("SELECT id FROM orderwise_subscribers WHERE orderwise_id = '" . $csvLine[1] . "'");
        $result = $handle->fetch_assoc();
        // No record exists
        if(empty($result)) {
            $mysqli->query("INSERT INTO orderwise_subscribers (orderwise_id, subscribed, email, status, import_into) VALUES ({$csvLine[1]}, 0, {$csvLine[0]}, {$status}, {$importInto})");
        }
        if($y % 1000 == 0) {
            echo "<pre>", print_r('memory usage : ' . memory_get_usage()), "</pre>";
        }
        $y++;
    }
    fclose($fp);
}