How to use PDO to close a connection in PHP

In modern development, the general use of frameworks will cause us to ignore the underlying encapsulation, and most frameworks have defaulted to using PDO for database operations. So everyone knows how PDO closes the data connection? ?

Official Description

To close the connection, you need to destroy the object to ensure that all remaining references to it are deleted, you can Assign a NULL value to the object variable. If you don't do this explicitly, PHP will automatically close the connection at the end of the script.

$pdo = new PDO('mysql:host=127.0.0.1;
port=3306;
dbname=blog_test', 'root', '');
$pdo = null;

The official documentation makes it very clear, that is, just assign NULL to the PDO object. But are things really that simple?

Actual test

Let’s do a test like this. Under normal circumstances, we will not close the database connection directly after opening it, but will need to perform some operations.

$pdo = new PDO('mysql:host=127.0.0.1;port=3306;dbname=blog_test', 'root', '');
$stmt = $pdo->prepare('SELECT * FROM zyblog_test_user');
$stmt->execute();
$pdo = null;
sleep(60);

After running the above code, we use show full processlist in the database; to view the connection process, we will find that the current connection is not closed immediately, but will not be closed until 60 seconds later, that is, after the page execution is completed. . It seems that $pdo = null; did not execute successfully.

Actually, this situation has been explained in the official documents, but people may not pay attention to it. [The object needs to be destroyed to ensure that all remaining references to it are deleted]. In the above code, the \$stmt precompiled SQL statement function calls the method in the $pdo object, creating a reference dependency between them. Relationship, in this case, directly giving $pdo = null; has no effect. We need to assign $stmt to null as well.

$pdo = new PDO('mysql:host=127.0.0.1;port=3306;dbname=blog_test', 'root', '');

$stmt = $pdo->prepare('SELECT * FROM zyblog_test_user');
$stmt->execute();

$stmt = null;
$pdo = null;
sleep(60);

mysqli test

So will there be this problem when using the default extension component of mysqli, that is, using close() in the mysqli object to close the database connection? Or test it directly with code. (The mysql extension is outdated and is not recommended. If you want to encapsulate the database operation class yourself or write a small Demo, it is better to use mysqli)

$conn = new mysqli('127.0.0.1', 'root', '', 'blog_test');

$result = $conn->query('SELECT * FROM zyblog_test_user');
$stmt = $conn->prepare("SELECT * FROM zyblog_test_user");
$stmt->execute();

$conn->close();

sleep(60);

After running the above code, we check the connection in the database The process will not see the connection that is still executing. In other words, calling the close() method in mysqli can directly close the database connection immediately.

Summary

In fact, today’s content is also the information in a Note on the official document about database connection. A master discovered this problem a long time ago and shared it, but most people don't know this problem at all, and many people even don't know that PDO can close the database connection.

While the framework brings us convenience, it also encapsulates many things so well that many friends don’t care about some of the underlying content. However, when you move to a higher level, Often these low-level things will become your obstacles.

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