PDO类与PDOStatement类之间的关系

Question

I'm a php and mysql beginner, I'm currently self study PDO and confused some concepts:

$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
$sql = "SELECT * FROM users";
$users = $dbh->query($sql);

1.What is the relationship between PDO class and PDOStatement class?

$dbh is the new object of class PDO, but why $users is the PDOStatement object? fetchAll() is the function inside class PDOStatement, but you can use it like this $users->fetchAll(), is $users a PDO or PDOStatement object?

2.Someone said $users is the cursor, once consumed, it won't rewind to the beginning of the resultset.

foreach ($users as $row) {
    print $row["name"] . "<br/>";
}

but why you can use it in a foreach statement? foreach provides a way to iterate over arrays. what is cursor actually? is cursor a pointer?

3.For the pdostatement class, the doc said:

PDOStatement implements Traversable { ... }

why this class implements Traversable interface? is it empty interface?

Thank you for help!

Answer 1

According to the documentation, the Traversable interface allows you to use the object into a foreach loop and it's only supposed to be used internally. Think of it as a convenient way of using the PDOStatement.

Basically, with PDO there is two ways to execute a query, one by using PDO::prepare() & PDOStatement::execute() and the other one by using PDO::query(). The later does prepare/execute in one call.

PDO::query() and PDO::execute() will not return the results on the other hand the PDOStatement object will allow you to specify the data you want to return. PDOStatement::fetchAll() will allow to define how you want to have your data organized.

It seems more complicated on first sight but it provides more flexibility.