使用PDO和MySql而不是mysqli [重复]

I'm starting using PDO and MySQL instead of mysqli. So, i try to define this simple function:

public function getIntro($table) {
            $stmt = $this->db->prepare("SELECT * FROM $table ORDER BY rand() LIMIT ?");
            $stmt->execute(array(6));
            return $stmt->fetchAll(PDO::FETCH_ASSOC);
        }

But, in doing so, the function return an empty array. Considering that the connection with the db work correctly, where is the error?

Thanks

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Try this:

public function getIntro($table) {
        $stmt = $this->db->prepare("SELECT * FROM $table ORDER BY rand() LIMIT ?");
        $stmt->bindValue(1, 6);
        $stmt->execute();
        return $stmt->fetchAll(PDO::FETCH_ASSOC);
    }

or this:

public function getIntro($table) {
            $stmt = $this->db->prepare("SELECT * FROM $table ORDER BY rand() LIMIT :limit");
            $stmt->execute(array(':limit' => 6));
            return $stmt->fetchAll(PDO::FETCH_ASSOC);
        }