将对象强制转换为扩展母类的子类

I am trying to add a method to a class by extending it and then to cast an object to my child extended class:

public class motherClass(){
  public function methodA(){}
}

public class childClass extends motherClass (){
  public function methodB(){}
}


$obj= Method that return an istance of motherClass
$objChild = (childClass) $obj;
$objChild->methodB();

Is possible to do this in PHP ? I need to instaziate the $objChild becouse I want to call some method from childClass in an object that belongs to motherClass.