When a PHP process connects to MySQL server, the connection can be persistent if your PHP config has mysql.allow_persistent or mysqli.allow_persistent. (PDO has the attribute ATTR_PERSISTENT)

$dbh = new PDO('DSN', 'KhanhDepZai', 'QuenMatKhauCMNR', [PDO::ATTR_PERSISTENT => TRUE]);

Object destruction

PHP destruct an object automatically when an object lost all its references.

Example code:


$x = null;

function klog($x) {
echo $x . ' => ';

class A {
private $k;
function __construct($k) {
$this->k = $k;

function b() {

function __destruct() {
klog("[{$this->k} has been killed]");

function c($k) {
return new A($k);

function d() {

function e() {
global $x;
$x = c('e');

function f() {



[b] => [d has been killed] => [b] => [e] => [f] => [e has been killed] =>

Reducing PDO persistent connections in PHP long-run process (connect to multiples databases)

Instead of using a service object, we should use a factory design pattern for each job (each connection). PHP will close MySQL connection because it destructs object PDO. Then we can reduce the number of connections to MySQL at a same time.

I learned this case when implement a web-consumer (long-run process) to run database migration for multiples databases.

Before fixing this, our MySQL server had been crashed because of a huge opened connections.

Now, everything works like a charm !

Bring it on