PHP里的单例类写法实例

php里的单实例类在进行数据交换，节省内存上还是很有意义的。写个简单例子。
类1，单实例类本身：

class util {
  private static $instance;
  public function get() {
    if (!self::$instance) {
      self::$instance = new util();
    }
    return self::$instance;
  }
  public $number = 10;
  public function change($num) {
    $this->number += $num;
  }
  public function getnum() {
    return $this->number;
  }
}

类2，使用前述单实例类的应用类：

class singlea {
  private $numinst;
  function __construct() {
    $this->numinst = util::get();
  }
  public function change($num) {
    $this->numinst->change($num);
  }
  public function getnum() {
    return $this->numinst->getnum();
  }
}

类3，同类2：

class singleb {
  private $numinst;
  function __construct() {
    $this->numinst = util::get();
  }
  public function change($num) {
    $this->numinst->change($num);
  }
  public function getnum() {
    return $this->numinst->getnum();
  }
}

最后是调用的地方：

$insta = new singlea();
$insta->change(100);
var_dump('singlea changed: ');
var_dump($insta->getnum());
$instb = new singleb();
$instb->change(-510);
var_dump('singleb changed: ');
var_dump($instb->getnum());

最后的显示结果：

string'singlea changed: ' (length=17)
int110
string'singleb changed: ' (length=17)
int-400