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