PDO：FetchAll返回空数组

I keep receiving an empty array when i try to execute the following code:

$this->DB->prepare('SELECT * FROM Articles WHERE Author = :username AND Timestamp < :timestamp ORDER BY Timestamp DESC LIMIT :limit;');
$this->DB->bind(':username', $username);
$this->DB->bind(':timestamp', $start);
$this->DB->bind(':limit', $numberOfResults, \PDO::PARAM_INT);
$data = $this->DB->execute();

These are the bind, prepare and execute functions:

public function bind($placeholder, $value, $type = \PDO::PARAM_STR) {

    $this->preparedQuery->bindParam($placeholder, $value, $type);
}

public function prepare($query) {

    if(!$this->preparedQuery = $this->connection->prepare($query)) {

        print_r($this->connection->errorInfo());
        die();
    }
}

public function execute() {

    if(!$this->preparedQuery->execute()) {

        print_r($this->preparedQuery->errorInfo());
        die();
    }

    return $this->preparedQuery->fetchAll(\PDO::FETCH_ASSOC);
}

I have also tryed to manually execute the query in MySQL Workbench and it's perfectly working.

Edit:

This is what the query looks like:

SELECT * FROM Articles WHERE Author = "TestUser" AND Timestamp < "9000-12-31 23:59:59" ORDER BY Timestamp DESC LIMIT 10;

Edit 2:

I just found out that if I remove the condition about the Timestamp the query returns values correctly.

I have finally come to a solution.

Thanks to Edit 2 i realized that, just like 90% of my errors, this was a stupid one.

The $start variable was actually empty even if in the code was correctly initialized in the function declaration. This was causing to have an empty string to be passed to the bindParam() function.

Personal comment: I am an idiot.