使用PDO从MySQL数据库获取信息

Question

I'm trying to connect to a MySQL database using PHP (with PDO) but I'm not that familiar with PHP so I'm a little stuck. I'm trying to search one of my databases for all occurrences of $userid and then store the $crn associated with it in an array but I don't really know how to go about that. Here's what I have so far...

$sthandler = $conn->prepare('SELECT userid FROM users WHERE userid=' . $userid);
    $sthandler->execute(array('username'));

    if($sthandler->rowCount() > 0)
    {
        // User exists, get user's courses from database and returns array of CRNs
        for(int i = 0; i < rowCount; i++)
        {
            $sthandler = $conn->prepare('SELECT crn FROM usercourses WHERE userid=' . $userid);
            $sthandler->execute(array($USER => $crnArray));
        }            
    }

Answer 1

The point of PDO is to prevent concatenating the query string like you're currently doing. It's supposed to be used used to prepare the statements.

$sthandler = $conn->prepare('SELECT userid FROM users WHERE userid= :user_id');
if($sthandler->execute(array(':user_id' => $user_id))) {
    if($sthandler->rowCount() > 0)
    {
        // User exists, get user's courses from database and returns array of CRNs
        for(int $i = 0; $i < rowCount; $i++)
        {
            $sthandler = $conn->prepare('SELECT crn FROM usercourses WHERE userid= :user_id';
            $sthandler->execute(array(':user_id' => $user_id));
        }            
    }
}

You're trying to execute with random data in the arrays.

One more thing, you should wrap your queries in a try/catch statement, to handle any potential errors:

try {
    // run query and all
}
catch (PDOException $e){
    // echo out the error if there is one.
    echo $e->getMessage();
}

And the last note, while you're at it. You should explicitly turn on errors when constructing your PDO object!

$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);