有人可以解释存储在“新PDO”中的内容[重复]

I am having difficult understanding the $conn within the tutorial I just did. It is from Tutsplus PHP Fundamentals course.

I have arrived at the following code:

<?php
require 'config.php';
$letter = 'J%';
try {
$conn = new PDO('mysql:host=localhost; dbname=practice', $config['DB_USERNAME'], $config['DB_PASSWORD']);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); //Switch to turn on error modes with errors displayed

$stmt = $conn->prepare('SELECT * FROM users WHERE username LIKE :placeholder');

$stmt->bindParam('placeholder', $letter, PDO::PARAM_INT);
$stmt->setFetchMode(PDO::FETCH_OBJ);
$stmt->execute();

while($row = $stmt->fetch()) {
    print_r($row);
}
} catch(PDOException $e) {
echo 'ERROR: ' . $e->getMessage();
}

I understand the :placeholder part. The part that I'm not quite grasping is particularly the $conn in this line:

$stmt = $conn->prepare('SELECT * FROM users WHERE username LIKE :placeholder');

If I'm understanding the -> now it means we're passing the variable $conn into the function prepare() and storing the result in $stmt. What I don't get is what $conn is currently holding from the PDO connection. I tried doing print $conn; but got returned an error.

</div>

$conn contains PDO class's object.

PDO (php database object) is a standart library to provide access to different database drivers using object-oriented interface.

"$conn" object is an instance of PDO class.

Objects in "object-oriented programming" are essentially data structures together with their associated processing routines.

You should learn at least OOP basics to fully understand how it works.

http://en.wikipedia.org/wiki/Object-oriented_programming

If I'm understanding the -> now it means we're passing the variable $conn into the function prepare()

No. "->" operator you calling a function "prepare" of the object "$conn"