流行php功能错误[关闭]

function insert_new_user($username, $password)
{
    # checking username is already taken
    if (username_exists($username))
        return false;

    # insert new user info
    global $pdo;
    $stmt = $pdo->pprepare('
        INSERT INTO users
        (username, password)
        values (:username, :password)');

    $stmt->execute( array(':username' => $username, ':password' => md5($password)) );

    if ($pdo->lastInsertId())
        return true;
    else
        return false;
}

i am getting uncertain error form this php codes when try to check user existence from mysql db. i didn't found the error. please help me if i am doing something wrong in coding.

function insert_new_user($username, $password)
{
    # checking username is already taken
    if (username_exists($username))
        return false;

    # insert new user info
    global $pdo;
    $stmt = $pdo->prepare('
        INSERT INTO users
        (username, password)
        values (:username, :password)');

    $stmt->execute( array(':username' => $username, ':password' => md5($password)) );

    if ($pdo->lastInsertId())
        return true;
    else
        return false;
}

try this one. i think u entered pp in prepare.