扩展另一个的PHP类如何继承私有函数？

<?
$a = new foo();
print $a->xxx();
$b = new bar();
print $b->xxx();

class foo {
  const something = 10;

  public function xxx() {
    $this->setSomething();
    return $this->something;
  }

  private function setSomething() {
    $this->something = self::something;
  }
}

class bar extends foo {

  public function xxx() {
    $this->setSomething();
    $this->something++;
    return $this->something;
  }

}
?>

但是当我运行脚本时,我收到以下错误：

Fatal error: Call to private method foo::setSomething() from context 'bar' in test.PHP on line 23

似乎bar不继承私有函数setSomething().如何在不修改foo类的情况下修复此问题？