Vlákna - kde se ztrácí připojení k DB?

<?php
$connection = new PDO('mysql:host=localhost;dbname=x', 'y', 'z');

class Model {
    public $connection;
    public function setConnection(PDO $connection) {
        $this->connection = $connection;
        //echo get_class($this->connection)." | setConnection<br />\n"; //zde ještě vrátí PDO
    }
}

class Program {
    public $model;
    public function setModel(Model $model) {
        $this->model = $model;
    }
    public function run() {
        echo get_class($this->model->connection)." | run<br />\n";
    }
}

class ProgramThread extends Thread {
    public $i;
    public $program;

    public function __construct($i, Program $program, PDO $connection)
    {
        $this->i = $i;
        $this->program = $program;
        $this->program->model->setConnection($connection);
        echo get_class($this->program->model->connection)." | __construct<br />\n";
    } 

    public function run()
    {
        $this->program->run();
    }
}

$program = new Program();
$model = new Model;//nemůžu předat připojení už teď do modelu, hlásilo by to nelze serializovat PDO
$program->setModel($model);

/*
1. varianta
$pt = new ProgramThread(1, clone $program, clone $connection);
$pt->run();
exit;
*/

//2. varianta
$threads = [];
foreach (range(1, 1) as $i) {
    $threads[$i] = new ProgramThread($i, clone $program, clone $connection);
    $threads[$i]->start();
}

Program | run
ProgramThread | __construct

PDO | __construct
PDO | run

<?php
class Program {
    public $connection;
    public function run() {
        echo get_class($this->connection)." | run<br />\n";
    }
}
class Connection { //neserializovatelná třída
}
class ProgramThread extends Thread { //zkuste odmazat  "extends Thread" a bude to fungovat dobře
    public $program;
    public function __construct(Program $program, Connection $connection)
    {
        $this->program = $program;
        //$connection je v pořádku, ale do $this->program->connection se nechce zapsat
        $this->program->connection = $connection; // $this->program->connection je i po tomhle NULL
        echo get_class($this->program->connection)." | __construct<br />\n";
    } 

    public function run()
    {
        $this->program->run();
    }
}

$connection = new Connection();
$program = new Program;

$pt = new ProgramThread(clone $program, clone $connection);
$pt->run();