Do SQL connections opened with PDO in PHP have to be closed

When I open a MySQL connection in PHP with just PHP's built-in MySQL functions, I do the following:

$link = mysql_connect($servername, $username, $password);
mysql_select_db($dbname);
//queries etcetera
mysql_close($link);

When I open a connection with PDO, it looks like this:

$link = new PDO("mysql:dbname=$dbname;host=$servername",$username,$password);
//prepare statements, perform queries

Do I have to explicitly close the connection like I do with mysql_connect() and mysql_close()? If not, how does PHP know when I'm done with my connection?

TIA.

Use $link = null to let PDO know it can close the connection.

PHP: PDO Connections & Connection Management

Upon successful connection to the database, an instance of the PDO class is returned to your script. The connection remains active for the lifetime of that PDO object. To close the connection, you need to destroy the object by ensuring that all remaining references to it are deleted--you do this by assigning NULL to the variable that holds the object. If you don't do this explicitly, PHP will automatically close the connection when your script ends.

PDO does not offer such a function on its own. Connections via PDO are indirectly managed via the PDO objects refcount in PHP.

But sometimes you want to close the connection anyway, regardless of the refcount. Either because you can not control it, need it for testing purposes or similar.

You can close the Mysql connection with PDO by running a SQL query. Every user that is able to connect to the Mysql server is able to KILL at least its own thread:

/*
 * Close Mysql Connection (PDO)
 */

$pdo_mysql_close = function (PDO $connection) {

    $query = 'SHOW PROCESSLIST -- ' . uniqid('pdo_mysql_close ', 1);
    $list  = $connection->query($query)->fetchAll(PDO::FETCH_ASSOC);
    foreach ($list as $thread) {
        if ($thread['Info'] === $query) {
            return $connection->query('KILL ' . $thread['Id']);
        }
    }
    return false;
};

$pdo_mysql_close($conn);

Related Mysql Documentation:

• 13.7.5.30. SHOW PROCESSLIST Syntax
• 13.7.6.4. KILL Syntax

Related Stackoverflow Questions:

• PHP PDO close()? (Apr 2012)