Save comment to external db as json

add_filter('preprocess_comment', 'check_is_private_message' );
function check_is_private_message( $commentdata ) {

    $is_private = (int)$_POST['is_private'];
    if ( $is_private ){
        //Remote mysql credentials
        $servername = "xxx.xx.xxx.xxx";
        $username = "testuser";
        $password = "testpass";
        $dbname = "testdb";
        // Create connection
        $conn = new mysqli($servername, $username, $password, $dbname);
        // Check connection
        if ($conn->connect_error) {
            die("Connection failed: " . $conn->connect_error);
        }
        // Save Json query
        $sql = "INSERT INTO yolo (yo) VALUES ('".mysql_real_escape_string(json_encode($_POST))."')";

        // Validate and debug
        $result = $conn->query($sql);
        if ($conn->query($sql) === TRUE) {
            echo "New record created successfully";
        } else {
            echo "Error: " . $sql . "<br>" . $conn->error;
        }
        $conn->close();
    }else{
        return $commentdata;
    }

}