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