Why PHP if..esleif..else starts from beginning again after MySQL query in it

I am using following code to run update query if value is not duplicate in database.

if($lastvote==false){ // There isn't duplicate entry, just run INSERT query.
    $this->save();
    return "Vote recorded!";
}elseif($lastvote=="down" && $this->vote_value=="down"){
    return "Already voted down!";
}elseif($lastvote=="up" && $this->vote_value=="up"){
    return "Already voted up!";
}elseif($lastvote=="up" && $this->vote_value=="down"){
    $this->update_last($this->topic_id, $this->updated_by, $this->vote_value);
    return "Vote changed to down!";
}elseif($lastvote=="down" && $this->vote_value=="up"){
    $this->update_last($this->topic_id, $this->updated_by, $this->vote_value);
    return "Vote changed to up!";
}

The queries works well. But after function

$this->update_last()

private function update_last($topic, $by, $value){
    global $database;
    $sql = "UPDATE ".static::$table_name." SET vote_value='".$value."' WHERE topic_id = ".$topic." AND updated_by = ".$by;
    $database->query($sql);
    return ($database->affected_rows() == 1) ? true : false;
}

it is not returning "Vote changed to up/down" as per last two conditions. Rather it returning "Already voted up/down" from first two queries after successfully executing the UPDATE query from last two conditions.

Please guide me where I am making mistake?