mysqli::prepare() [mysqli.prepare]: All data must be fetched before a new statement prepare takes place

I want to fetch entries (by different persons), from a table. Now I want to look for each of the fetched person's name in another table and see if he has any photo. My code is given below, however its not working, I'm getting following errors:

mysqli::prepare() [mysqli.prepare]: All data must be fetched before a new statement prepare takes place
    Call to a member function fetch() on a non-object in ...

My Code:

if ($stmt = $this->mysqli->prepare("SELECT entry, author, time FROM messages WHERE user = ?")) {
    $stmt->bind_param("s", $user_name);
    $stmt->execute();
    $stmt->bind_result($entry, $author, $time);

    while ($stmt->fetch()) {
        if ($stmt = $this->mysqli->prepare("SELECT photo_id FROM photos WHERE user = ?")) {
            $stmt->bind_param("s", $author);
            $stmt->execute();
            $stmt->bind_result($photo_id);
        }
        //echo $photo_id;
    }
    $stmt->close();
}

Assign the second statement to new variable so it wouldn't override the first variable and cause the "all data must be fetched.." error.

if ($stmt = $this->mysqli->prepare("SELECT entry, author, time FROM messages WHERE user = ?")) {
        $stmt->bind_param("s", $user_name);
        $stmt->execute();
        $stmt->bind_result($entry, $author, $time);

        while ($stmt->fetch()) {             
            if ($st = $this->mysqli->prepare("SELECT photo_id FROM photos WHERE user = ?")) {
                $st->bind_param("s", $author);
                $st->execute();   
                $st->bind_result($photo_id); 
            }
            //echo $photo_id;
            $st->close();
        }    
    $stmt->close();
}