Finesse

public function doSomething1() {/*...*/}
public function doSomething2() {/*...*/}

class CommonClass {
  public function foo() {/*...*/}
  public function bar() {/*...*/}
}

class ConcreteClass1 extends CommonClass {
  include "common.php";
}

class ConcreteClass2  {
  include "common.php";
  public function doSomething3() {/*...*/}
}