mysqli connection in CLOSE_WAIT state when forked

<?php

$asyncTest = new asyncTest();
$asyncTest->treatElement();

class asyncTest
{
  private $db;
  public function __construct()
  {
    echo "New asyncTest (" . getmypid() . ")\n";
    $this->db = mysqli_init();
    $this->db->real_connect('127.0.0.1', 'user', 'pass', 'db');
  }

  public function __destruct()
  {
    $this->disconnect();
  }

  public function disconnect()
  {
    if(isset($this->db))
    {
      $threadId = $this->db->thread_id;
      if($threadId)
      {
        $this->db->kill($threadId);
      }
      $this->db->close();
      unset($this->db);
    }
  }

  public function treatElement()
  {
    sleep(1);
    //$this->disconnect();
    exec("php -d display_errors=on -f conn.php >> conn.log 2>&1  < /dev/null &");
  }

}