class YourFirstClass {
   public $variable;
   private $_variable2;
   public function setVariable2($a) {
       $this->_variable2 = $a;
   }
}
 
class YourSecondClass {
   public $variable;
   private $_variable2;
   public function setVariable2($a) {
       $this->_variable2 = $a;
   }
}
 
class ContaingClass {
   private $_first;
   private $_second;
   public function __construct(YourFirstClass $first,YourSecondClass $second) {
       $this->_first = $first;
       $this->_second = $second;
   }
   public function doSomething($aa) {
       $this->_first->setVariable2($aa);
   }
}

研究(谷歌)：“继承的构成”

脚注：抱歉非创意变量名称..